Display device and manufacturing method of the same

ABSTRACT

A novel display device that is highly convenient or reliable is provided. Furthermore, a display device with low power consumption and high display quality is provided. The display device includes a first display element, a second display element, a first transistor, and a second transistor. The first display element includes a first pixel electrode and a liquid crystal layer. The second display element includes a second pixel electrode and a light-emitting layer. The first transistor is electrically connected to the first pixel electrode. The second transistor is electrically connected to the second pixel electrode. The first transistor and the second transistor each include an oxide semiconductor film. The first pixel electrode and the second pixel electrode each include at least one metal element contained in the oxide semiconductor film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a display device anda manufacturing method thereof.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specific examples of the technical field ofone embodiment of the present invention disclosed in this specificationinclude a semiconductor device, a display device, a light-emittingdevice, a power storage device, a memory device, a method for drivingany of them, and a method for manufacturing any of them.

2. Description of the Related Art

There is a liquid crystal display device in which a surface-emittinglight source is provided as a backlight and combined with a transmissiveliquid crystal display device in order to reduce power consumption andsuppress a reduction in display quality (see Patent Document 1).

REFERENCE Patent Document [Patent Document 1] Japanese Published PatentApplication No. 2011-248351 SUMMARY OF THE INVENTION

An object of one embodiment of the present invention is to provide anovel display device that is highly convenient or reliable.

Another object of one embodiment of the present invention is to providea display device with low power consumption and high display quality.Another object of one embodiment of the present invention is to reducethe width of the driver circuit of the display device to achieve anarrower bezel. Another object of one embodiment of the presentinvention is to provide a novel display device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention is a display device including afirst display element, a second display element, a first transistor, anda second transistor. The first display element includes a first pixelelectrode and a liquid crystal layer. The second display elementincludes a second pixel electrode and a light-emitting layer. The firsttransistor is electrically connected to the first pixel electrode. Thesecond transistor is electrically connected to the second pixelelectrode. The first transistor and the second transistor each includean oxide semiconductor film. The first pixel electrode and the secondpixel electrode each include at least one metal element contained in theoxide semiconductor film.

In the above embodiment, it is preferable that the first display elementfurther include a reflective film. In the above embodiment, it ispreferable that the reflective film be electrically connected to thefirst pixel electrode and have a function of reflecting incident light.In the above embodiment, it is preferable that the reflective film beprovided with an opening transmitting incident light. In the aboveembodiment, it is preferable that the second display element have afunction of emitting light toward the opening.

In any of the above embodiments, the oxide semiconductor film preferablycontains In, Zn, and M (M is Al, Ga, Y, or Sn). In any of the aboveembodiments, it is preferable that the oxide semiconductor film includea crystal part and that the crystal part have c-axis alignment.

In any of the above embodiments, it is preferable that one or both ofthe first transistor and the second transistor have a staggeredstructure.

In any of the above embodiments, it is preferable that one or both ofthe first transistor and the second transistor have a first conductivefilm, a first insulating film over the first conductive film, a firstoxide semiconductor film over the first insulating film, a secondinsulating film over the first oxide semiconductor film, and a secondoxide semiconductor film over the second insulating film. The secondoxide semiconductor film covers a side surface of the first oxidesemiconductor film with the second insulating film positionedtherebetween in a cross section in a channel width direction. The firstoxide semiconductor film is surrounded by the first conductive film andthe second oxide semiconductor film in the cross section in the channelwidth direction.

One embodiment of the present invention can provide a novel displaydevice that is highly convenient or reliable. According to oneembodiment of the present invention, a display device with low powerconsumption and high display quality can be provided. According to oneembodiment of the present invention, a display device with reduced widthof the driver circuit and a narrower bezel can be provided. According toone embodiment of the present invention, a novel display device can beprovided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram and circuit diagrams illustrating a displaydevice.

FIG. 2 is a circuit diagram illustrating a pixel circuit.

FIGS. 3A and 3B are schematic views each illustrating a display regionof a display element.

FIGS. 4A and 4B are top views illustrating a display device and a pixelcircuit.

FIG. 5 is a cross-sectional view illustrating a display device.

FIG. 6 is a cross-sectional view illustrating a display device.

FIG. 7 is a cross-sectional view illustrating a display device.

FIG. 8 is a cross-sectional view illustrating a display device.

FIGS. 9A to 9C illustrate cross-sectional views illustrating amanufacturing process of a display device.

FIGS. 10A to 10C are cross-sectional views illustrating a manufacturingprocess of a display device.

FIGS. 11A to 11C are cross-sectional views illustrating a manufacturingprocess of a display device.

FIGS. 12A to 12C are cross-sectional views illustrating a manufacturingprocess of a display device.

FIGS. 13A and 13B are cross-sectional views illustrating a manufacturingprocess of a display device.

FIG. 14 is a cross-sectional view illustrating a manufacturing processof a display device.

FIG. 15 is a cross-sectional view illustrating a display device.

FIG. 16 is a cross-sectional view illustrating a display device.

FIG. 17 is a cross-sectional view illustrating a display device.

FIG. 18 is a cross-sectional view illustrating a display device.

FIGS. 19A to 19C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 20A to 20C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 21A and 21B are cross-sectional views illustrating a semiconductordevice.

FIGS. 22A and 22B are cross-sectional views illustrating a semiconductordevice.

FIGS. 23A and 23B are cross-sectional views illustrating a semiconductordevice.

FIGS. 24A and 24B are cross-sectional views illustrating a semiconductordevice.

FIGS. 25A and 25B are cross-sectional views illustrating a semiconductordevice.

FIGS. 26A and 26B are cross-sectional views illustrating a semiconductordevice.

FIGS. 27A and 27B are cross-sectional views illustrating a semiconductordevice.

FIGS. 28A and 28B are cross-sectional views illustrating a semiconductordevice.

FIGS. 29A and 29B are cross-sectional views illustrating a semiconductordevice.

FIGS. 30A to 30C each illustrate a band structure.

FIGS. 31A to 31D are cross-sectional views illustrating a method formanufacturing a semiconductor device.

FIGS. 32A to 32C are cross-sectional views illustrating a method formanufacturing a semiconductor device.

FIGS. 33A and 33B are cross-sectional views illustrating a method formanufacturing a semiconductor device.

FIGS. 34A to 34D are cross-sectional views illustrating a method formanufacturing a semiconductor device.

FIGS. 35A to 35C are cross-sectional views illustrating a method formanufacturing a semiconductor device.

FIGS. 36A to 36C are cross-sectional views illustrating a method formanufacturing a semiconductor device.

FIGS. 37A to 37E show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD and selected-area electrondiffraction patterns of a CAAC-OS.

FIGS. 38A to 38E show a cross-sectional TEM image and plan-view TEMimages of a CAAC-OS and images obtained through analysis thereof.

FIGS. 39A to 39D show electron diffraction patterns and across-sectional TEM image of an nc-OS.

FIGS. 40A and 40B are cross-sectional TEM images of an a-like OS.

FIG. 41 shows a change of crystal parts of an In—Ga—Zn oxide due toelectron irradiation.

FIG. 42 illustrates a display module.

FIGS. 43A to 43E each illustrate an electronic device.

FIGS. 44A to 44E are perspective views each illustrating a displaydevice.

FIGS. 45A and 45B are perspective views illustrating a display device.

FIGS. 46A and 46B illustrate a structure of a data processor.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described below with reference to drawings. However,the embodiments can be implemented in many different modes, and it willbe readily appreciated by those skilled in the art that modes anddetails thereof can be changed in various ways without departing fromthe spirit and scope of the present invention. Thus, the presentinvention should not be interpreted as being limited to the followingdescription of the embodiments.

In the drawings, the size, the layer thickness, and the region areexaggerated for clarity in some cases. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematic views showing ideal examples, and embodiments ofthe present invention are not limited to shapes or values shown in thedrawings.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Note that in this specification, terms for describing arrangement, suchas “over” “above”, “under”, and “below”, are used for convenience indescribing a positional relation between components with reference todrawings. Furthermore, the positional relation between components ischanged as appropriate in accordance with a direction in which eachcomponent is described. Thus, there is no limitation on terms used inthis specification, and description can be made appropriately dependingon the situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. In addition, thetransistor has a channel region between a drain (a drain terminal, adrain region, or a drain electrode) and a source (a source terminal, asource region, or a source electrode), and current can flow through thedrain, the channel region, and the source. Note that in thisspecification and the like, a channel region refers to a region throughwhich current mainly flows.

Furthermore, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, the expression“electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on an “object having any electric function” aslong as electric signals can be transmitted and received betweencomponents that are connected through the object. Examples of an “objecthaving any electric function” are a switching element such as atransistor, a resistor, an inductor, a capacitor, and elements with avariety of functions as well as an electrode and a wiring.

In this specification and the like, the term “parallel” indicates thatthe angle formed between two straight lines is greater than or equal to−10° and less than or equal to 10°, and accordingly also includes thecase where the angle is greater than or equal to −5° and less than orequal to 5°. The term “perpendicular” indicates that the angle formedbetween two straight lines is greater than or equal to 80° and less thanor equal to 100°, and accordingly also includes the case where the angleis greater than or equal to 85° and less than or equal to 95°.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other. For example, in some cases, the term“conductive film” can be used instead of the term “conductive layer”,and the term “insulating layer” can be used instead of the term“insulating film”.

Unless otherwise specified, off-state current in this specification andthe like refers to drain current of a transistor in an off state (alsoreferred to as a non-conducting state and a cutoff state). Unlessotherwise specified, the off state of an n-channel transistor means thatthe voltage between its gate and source (V_(gs): gate-source voltage) islower than the threshold voltage V_(th), and the off state of ap-channel transistor means that the gate-source voltage V_(gs) is higherthan the threshold voltage V_(th). For example, the off-state current ofan n-channel transistor sometimes refers to drain current that flowswhen the gate-source voltage V_(gs) is lower than the threshold voltageV_(th).

The off-state current of a transistor depends on V_(gs) in some cases.Therefore, “the off-state current of a transistor is I or lower” maymean that the off-state current of the transistor is I or lower at acertain V_(gs). The off-state current of a transistor may refer tooff-state current at a given V_(gs), at V_(gs) in a given range, atV_(gs) at which sufficiently low off-state current is obtained, or thelike.

As an example, an assumption is made that an n-channel transistor has athreshold voltage V_(th) of 0.5 V and a drain current of 1×10⁻⁹ A atV_(gs) of 0.5 V, 1×10⁻¹³ A at V_(gs) of 0.1 V, 1×10⁻¹⁹ A at V_(gs) of−0.5 V, and 1×10⁻²² A at V_(gs) of −0.8 V. The drain current of thetransistor is 1×10⁻¹⁹ A or lower at V_(gs) of −0.5 V or at V_(gs) in therange of −0.8 V to −0.5 V; therefore, it may be said that the off-statecurrent of the transistor is 1×10⁻¹⁹ A or lower. Since the drain currentof the transistor is 1×10⁻²² A or lower at a certain V_(gs), it may besaid that the off-state current of the transistor is 1×10⁻²² A or lower.

In this specification and the like, the off-state current of atransistor with a channel width W is sometimes represented by a currentvalue per channel width W or by a current value per given channel width(e.g., 1 μm). In the latter case, the off-state current may berepresented by current per length (e.g., A/μm).

The off-state current of a transistor depends on temperature in somecases. Unless otherwise specified, the off-state current in thisspecification may be off-state current at room temperature, 60° C., 85°C., 95° C., or 125° C. Alternatively, the off-state current may beoff-state current at a temperature at which the reliability of asemiconductor device or the like including the transistor is ensured ora temperature at which the semiconductor device or the like includingthe transistor is used (e.g., a temperature in the range of 5° C. to 35°C.). The state in which the off-state current of a transistor is I orlower may indicate that the off-state current of the transistor at roomtemperature, 60° C., 85° C., 95° C., 125° C., a temperature at which thereliability of a semiconductor device or the like including thetransistor is ensured, or a temperature at which the semiconductordevice or the like including the transistor is used (e.g., a temperaturein the range of 5° C. to 35° C.) is I or lower at a certain V_(gs).

The off-state current of a transistor depends on the voltage V_(ds)between its drain and source in some cases. Unless otherwise specified,the off-state current in this specification may be off-state current atV_(ds) of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12V, 16 V, or 20 V. Alternatively, the off-state current may be off-statecurrent at V_(ds) at which the reliability of a semiconductor device orthe like including the transistor is ensured or at V_(ds) used in thesemiconductor device or the like including the transistor. The state inwhich the off-state current of a transistor is I or lower may indicatethat the off-state current of the transistor at V_(ds) of 0.1 V, 0.8 V,1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V, atV_(ds) at which the reliability of a semiconductor device or the likeincluding the transistor is ensured, or at V_(ds) used in thesemiconductor device or the like including the transistor is I or lowerat a certain V_(gs).

In the above description of the off-state current, a drain may bereplaced with a source. That is, the off-state current sometimes refersto current that flows through a source of a transistor in the off state.

In this specification and the like, the term “leakage current” sometimesexpresses the same meaning as “off-state current”. In this specificationand the like, the off-state current sometimes refers to current thatflows between a source and a drain of a transistor in the off state, forexample.

In this specification and the like, a “semiconductor” includescharacteristics of an “insulator” in some cases when the conductivity issufficiently low, for example. Furthermore, a “semiconductor” and an“insulator” cannot be strictly distinguished from each other in somecases because a border between the “semiconductor” and the “insulator”is not clear. Accordingly, a “semiconductor” in this specification andthe like can be called an “insulator” in some cases. Similarly, an“insulator” in this specification and the like can be called a“semiconductor” in some cases. Alternatively, an “insulator” in thisspecification and the like can be called a “semi-insulator” in somecases.

In this specification and the like, a “semiconductor” includescharacteristics of a “conductor” in some cases when the conductivity issufficiently high, for example. Further, a “semiconductor” and a“conductor” cannot be strictly distinguished from each other in somecases because a border between the “semiconductor” and the “conductor”is not clear. Accordingly, a “semiconductor” in this specification andthe like can be called a “conductor” in some cases. Similarly, a“conductor” in this specification and the like can be called a“semiconductor” in some cases.

In this specification and the like, an impurity in a semiconductorrefers to an element that is not a main component of the semiconductor.For example, an element with a concentration of lower than 0.1 atomic %is an impurity. When an impurity is contained, the density of states(DOS) may be formed in a semiconductor, the carrier mobility may bedecreased, or the crystallinity may be decreased, for example. In thecase where the semiconductor includes an oxide semiconductor, examplesof an impurity which changes characteristics of the semiconductorinclude Group 1 elements, Group 2 elements, Group 14 elements, Group 15elements, and transition metals other than the main components;specifically, there are hydrogen (included in water), lithium, sodium,silicon, boron, phosphorus, carbon, and nitrogen, for example. In thecase of an oxide semiconductor, oxygen vacancy may be formed by entry ofimpurities such as hydrogen. Furthermore, when the semiconductorincludes silicon, examples of an impurity which changes thecharacteristics of the semiconductor include oxygen, Group 1 elementsexcept hydrogen, Group 2 elements, Group 13 elements, and Group 15elements.

Embodiment 1

In this embodiment, a display device of one embodiment of the presentinvention will be described with reference to FIGS. 1, 2, 3A and 3B, 4Aand 4B, 5, 6, 7, 8, 9A to 9C, 10A to 10C, 11A to 11C, 12A to 12C, 13Aand 13B, and 14.

<1-1. Structure of Display Device>

First, the structure of a display device is described with reference toFIG. 1. A display device 500 illustrated in FIG. 1 includes a pixelportion 502, gate driver circuit portions 504 a and 504 b and a sourcedriver circuit portion 506 placed outside the pixel portion 502.

[Pixel Portion]

The pixel portion 502 includes pixel circuits 501 (X, Y) arranged in Xrows (X is a natural number of 2 or more) and Y columns (Y is a naturalnumber of 2 or more). The pixel circuits 501 (X, Y) each include twodisplay elements which have different functions. One of the two displayelements has a function of reflecting incident light, and the other hasa function of emitting light. Note that the details of the two displayelements are described later.

[Gate Driver Circuit Portion]

Some or all of the gate driver circuit portions 504 a and 504 b and thesource driver circuit portion 506 are preferably formed over a substrateover which the pixel portion 502 is formed. Thus, the number ofcomponents and the number of terminals can be reduced. In the case wheresome or all of the gate driver circuit portions 504 a and 504 b and thesource driver circuit portion 506 are not formed over the substrate overwhich the pixel portion 502 is formed, a separately prepared drivercircuit board (e.g., a driver circuit board formed using a singlecrystal semiconductor film or a polycrystalline semiconductor film) maybe formed in the display device 500 by chip on glass (COG) or tapeautomated bonding (TAB).

The gate driver circuit portions 504 a and 504 b have a function ofoutputting a signal (a scan signal) for selecting the pixel circuits 501(X, Y). The source driver circuit portion 506 has a function ofsupplying a signal (data signal) for driving the display elementsincluded in the pixel circuits 501 (X, Y).

The gate driver circuit portion 504 a has a function of controlling thepotentials of wirings supplied with scan signals (hereinafter, suchwirings are referred to as scan lines G_(E) _(_) ₁ to G_(E) _(_) _(X))or a function of supplying an initialization signal. The gate drivercircuit portion 504 b has a function of controlling the potentials ofwirings supplied with scan signals (hereinafter, such wirings arereferred to as scan lines G_(L) _(_) ₁ to G_(L) _(_) _(X)) or a functionof supplying an initialization signal. Without being limited thereto,the gate driver circuit portions 504 a and 504 b each can control orsupply another signal.

Although the structure in which the two gate driver circuit portions 504a and 504 b are provided as gate driver circuit portions is illustratedin FIG. 1, the number of the gate driver circuit portions is not limitedthereto, and one or three or more gate driver circuit portions may beprovided.

[Source Driver Circuit Portion]

The source driver circuit portion 506 has a function of generating adata signal to be written to the pixel circuits 501 (X, Y) on the basisof an image signal, a function of controlling the potentials of wiringssupplied with data signals (such wirings are hereinafter referred to assignal lines S_(L) _(_) ₁ to S_(L) _(_) _(Y) and signal lines S_(E) _(_)₁ to S_(E) _(_) _(Y)), or a function of supplying an initializationsignal. Without being limited thereto, the source driver circuit portion506 may have a function of generating, controlling, or supplying anothersignal.

The source driver circuit portion 506 includes a plurality of analogswitches or the like. The source driver circuit portion 506 can output,as data signals, time-divided image signals obtained by sequentiallyturning on the plurality of analog switches.

Although the structure where one source driver circuit portion 506 isprovided is illustrated in FIG. 1, the number of the source drivercircuit portions is not limited thereto, and a plurality of sourcedriver circuit portions may be provided in the display device 500. Forexample, two source driver circuit portions may be provided so that thesignal lines S_(L 1) to S_(L Y) are controlled by one of the sourcedriver circuit portions and the signal lines S_(E) _(_) ₁ to S_(E) _(_)_(Y) are controlled by the other of the source driver circuit portions.

[Pixel Circuit]

A pulse signal is input to each of the pixel circuits 501 (X, Y) throughone of the scan lines G_(L 1) to G_(L X) and the scan lines G_(E 1) toG_(E X). A data signal is input to each of the pixel circuits 501 (X, Y)through one of the signal lines S_(L) _(_) ₁ to S_(L) _(_) _(Y) and thesignal lines S_(E) _(_) ₁ to S_(E) _(_) _(Y).

For example, the pixel circuit 501 (m, n) in the m-th row and the n-thcolumn (m is a natural number of X or less, and n is a natural number ofY or less) is supplied with pulse signals from the gate driver circuitportion 504 a through the scan lines G_(L) _(_) _(m) and G_(E) _(_) _(m)and supplied with a data signal from the source driver circuit portion506 through the signal lines S_(L) _(_) _(n) and S_(E) _(_) _(n) inaccordance with the potentials of the scan lines G_(L) _(_) _(m) andG_(E) _(_) _(m).

The pixel circuit 501 (m, n) includes two display elements as describedabove. The scan lines G_(L) _(_) ₁ to G_(L) _(_) _(X) are wirings whichcontrol a potential of a pulse signal supplied to one of the two displayelements. The scan lines G_(E 1) to G_(E X) are wirings which control apotential of the pulse signal supplied to the other of the two displayelements.

The signal lines S_(L) _(_) ₁ to S_(L) _(_) _(Y) are wirings whichcontrol a potential of a data signal supplied to one of the two displayelements. The signal lines S_(E) _(_) ₁ to S_(E) _(_) _(Y) are wiringswhich control the potential of a data signal supplied to the other ofthe two display elements.

[External Circuit]

External circuits 508 a and 508 b are connected to the display device500. Note that the external circuits 508 a and 508 b may be formed inthe display device 500.

As shown in FIG. 1, the external circuit 508 a is electrically connectedto wirings supplied with anode potentials (hereinafter referred to asanode lines ANO_(—1) to ANO_(X)) and the external circuit 508 b iselectrically connected to wirings supplied with common potentials(hereinafter referred to as common lines COM_(—1) to COM_(—X)).

<1-2. Configuration Example of Pixel Circuit>

Next, the description is made of a structure of the pixel circuit 501(m, n) with reference to FIG. 2.

FIG. 2 is a circuit diagram showing the pixel circuit 501 (m, n) and anadjacent pixel circuit (m, n+1) in a column direction of the pixelcircuit 501 (m, n) which are included in the display device 500 of oneembodiment of the present invention. In this specification and the like,the column direction is a direction in which the value of n of thesignal line S_(L n) (or the signal line S_(E n)) increases or decreasesand the row direction is a direction in which the value of m of the scanline G_(L) _(_) _(m) (or the scan line G_(E) _(_) _(m)) increases anddecreases.

The pixel circuit 501 (m, n) includes a transistor Tr1, a transistorTr2, a transistor Tr3, a capacitor C1, a capacitor C2, a display element430, and a display element 630. The pixel circuit (m, n+1) has aconfiguration similar to that of the pixel circuit 501 (m, n).

The pixel circuit 501 (m, n) is electrically connected to the signalline S_(L) _(_) _(n), the signal line S_(E) _(_) _(n), the scan lineG_(L) _(_) _(m), the scan line G_(E) _(_) _(m), a common line COM_(—m),a common line VCOM1, a common line VCOM2, and an anode line ANO_(—m).The pixel circuit 501 (m, n+1) is electrically connected to a signalline S_(L) _(_) _(n+1), a signal line S_(E) _(_) _(n+1), the scan lineG_(L) _(_) _(m), the scan line G_(E) _(_) _(m), the common lineCOM_(—m), the common line VCOM1, the common line VCOM2, and an anodeline ANO_(—m).

Each of the signal lines S_(L) _(_) _(n) and S_(L) _(_) _(n+1), the scanline G_(L) _(_) _(m), the common line COM_(—m), and the common lineVCOM1 is a wiring for driving the display element 430. Each of thesignal lines S_(E) _(_) _(n) and S_(E) _(_) _(n+1), the scan line G_(E)_(_) _(m), the common line VCOM2, and the anode line ANO_(—m) is awiring for driving the display element 630.

In the case where a potential supplied to the signal line S_(E) _(_)_(n) and the signal line S_(E) _(_) _(n+1) is different from a potentialsupplied to the signal line S_(L) _(_) _(n) and the signal lineS_(L n+1), the signal line S_(E n) and the signal line S_(L n+1) arepreferably positioned apart from each other as shown in FIG. 2. In otherwords, the signal line S_(E) _(_) _(n) is preferably positioned adjacentto the signal line S_(E) _(_) _(n+1). With this arrangement, aninfluence of the potential difference between the signal lines S_(L)_(_) _(n) and S_(L) _(_) _(n+1) and signal lines S_(E) _(_) _(n) andS_(E) _(_) _(n+1) can be reduced.

<1-3. Structure Example of First Display Element>

The display element 430 has a function of controlling transmission orreflection of light. In particular, the display element 430 ispreferably a so-called reflective display element which controlsreflection of light. The display element 430 serving as a reflectivedisplay element can reduce power consumption of the display devicebecause display can be performed with the use of external light. Forexample, the display element 430 may have a combined structure of areflective film, a liquid crystal element, and a polarizing plate, astructure using a micro electro mechanical systems (MEMS), or the like.

<1-4. Structure Example of Second Display Element>

The display element 630 has a function of outputting light, that is,emitting light. Therefore, the display element 630 may be rephrased as alight-emitting element. For example, an electroluminescence element(also referred to as an EL element), or a light-emitting diode may beused as the display element 630.

As described above, in the display device of one embodiment of thepresent invention, display elements with different functions are used asshown in the display elements 430 and 630. In the case where areflective liquid crystal element is used as one of the display elementsand a transmissive EL element is used as the other of the displayelements, a novel display device that is highly convenient or reliablecan be provided. Furthermore, a display device with low powerconsumption and high display quality can be provided when a reflectiveliquid crystal element is used in an environment with bright externallight and a transmissive EL element is used in an environment with weakexternal light.

<1-5. Driving Method for Display Element>

Next, a method for driving the display element 430 and the displayelement 630 is described. Note that a structure including a liquidcrystal element as the display element 430 and a light-emitting elementas the display element 630 is used in the description below.

<Driving Method for First Display Element>

A gate electrode of the transistor Tr1 is electrically connected to thescan line G_(L) _(_) _(m) in the pixel circuit 501 (m, n). One of asource electrode and a drain electrode of the transistor Tr1 iselectrically connected to the signal line S_(L) _(_) _(n). The other ofthe source electrode and the drain electrode of the transistor Tr1 iselectrically connected to one of a pair of electrodes of the displayelement 430. The transistor Tr1 is configured to be turned on or off tocontrol whether a data signal is written.

The other of the pair of electrodes of the display element 430 iselectrically connected to the common line VCOM1.

One of a pair of electrodes of the capacitor C1 is electricallyconnected to the common line COM_(—m), and the other of the pair ofelectrodes of the capacitor C1 is electrically connected to the other ofthe source electrode and the drain electrode of the transistor Tr1 andone of the pair of electrodes of the display element 430. The capacitorC1 has a function of storing data written to the pixel circuit 501 (m,n).

For example, the gate driver circuit portion 504 b in FIG. 1sequentially selects the pixel circuits row by row to turn on thetransistor Tr1, and data of data signals are written. When thetransistor Tr1 is turned off, the pixel circuit 501 (m, n) to which thedata has been written is brought into a holding state. This operation issequentially performed row by row; thus, an image is displayed.

[Driving Method for Second Display Element]

A gate electrode of the transistor Tr2 is electrically connected to thescan line G_(E) _(_) _(m) in the pixel circuit 501 (m, n). One of asource electrode and a drain electrode of the transistor Tr2 iselectrically connected to the signal line S_(E) _(_) _(n) and the otherof the source electrode and the drain electrode is electricallyconnected to a gate electrode of the transistor Tr3. The transistor Tr2is configured to be turned on or off to control whether a data signal iswritten.

One of a pair of electrodes of the capacitor C2 is electricallyconnected to the anode line ANO_(m). The other of the pair of electrodesof the capacitor C2 is electrically connected to the other of the sourceelectrode and the drain electrode of the transistor Tr2. The capacitorC2 has a function of storing data written to the pixel circuit 501 (m,n).

The gate electrode of the transistor Tr3 is electrically connected tothe other of the source electrode and the drain electrode of thetransistor Tr2. One of a source electrode and a drain electrode of thetransistor Tr3 is electrically connected to the anode line ANO_(—m). Theother of the source electrode and the drain electrode of the transistorTr3 is electrically connected to one of a pair of electrodes of thedisplay element 630. The transistor Tr3 includes a backgate electrode.The backgate electrode is electrically connected to the gate electrodeof the transistor Tr3.

The other of the pair of electrodes of the display element 630 iselectrically connected to the common line VCOM2.

For example, the gate driver circuit portion 504 a in FIG. 1sequentially selects the pixel circuits row by row to turn on thetransistor Tr2, and data of data signals are written. When thetransistor Tr2 is turned off, the pixel circuit 501 (m, n) to which thedata has been written is brought into a holding state. Furthermore, theamount of current flowing between the source electrode and the drainelectrode of the transistor Tr3 is controlled in accordance with thepotential of the written data signal. The display element 630 emitslight with a luminance corresponding to the amount of flowing current.This operation is sequentially performed row by row; thus, an image isdisplayed.

In this manner, two display elements can be controlled separately withthe use of different transistors in the display device of one embodimentof the present invention. Accordingly, a display device having highdisplay quality can be provided.

Transistors used in the display device of one embodiment of the presentinvention (the transistor Tr1 and the transistor Tr2) each include anoxide semiconductor film. The transistor including an oxidesemiconductor film can have relatively high field-effect mobility andthus can operate at high speed. The off-state current of the transistorincluding an oxide semiconductor film is extremely low. Therefore, theluminance of the display device can be kept even when the refresh rateof the display device is lowered, so that power consumption can belowered.

A progressive type display, an interlace type display, or the like canbe employed as the display type of the display element 430 and thedisplay element 630.

Further, as color elements controlled in the pixel at the time of colordisplay, three colors of R (red), G (green), and B (blue) can be given.Note that color elements are not limited to the three colors of R, G,and B. Alternatively, one or more colors of yellow, cyan, magenta,white, and the like may be added to RGB. Further, the sizes of displayregions may be different between respective dots of color elements.However, the display device of one embodiment of the present inventionis not limited to a color display device and can be applied to amonochrome display device.

<1-6. Display Region of Display Element>

Here, the display regions of the display elements 430 and 630 in thepixel circuit 501 (m, n) are illustrated using FIGS. 3A and 3B.

FIG. 3A is a schematic view illustrating display regions of the pixelcircuit 501 (m, n) and pixel circuits 501 (m, n−1) and 501 (m, n+1)which are adjacent to the pixel circuit 501 (m, n) in the columndirection.

The pixel circuit 501 (m, n), the pixel circuit 501 (m, n−1), and thepixel circuit 501 (m, n+1) illustrated in FIG. 3A each include a displayregion 430 d functioning as a display region of the display element 430and a display region 630 d functioning as a display region of thedisplay element 630.

For example, the display region 430 d includes a region which reflectslight and the display region 630 d includes a region which transmitslight. Furthermore, as shown in FIG. 3A, each of the pixel circuits 501(m, n−1) and (m, n+1) adjacent to the pixel circuit 501 (m, n) in thecolumn direction of the pixel circuit 501 (m, n) preferably includes thedisplay region 630 d at a position different from the position of thedisplay region 630 d in the pixel circuit 501 (m, n).

With the arrangement of the display regions 630 d shown in FIG. 3A, themanufacturing yield in the case of separately forming the displayelement 630 can be increased or interference of light extracted from thedisplay elements 630 between adjacent pixel circuits can be suppressed.

Although an example where the pixel circuits 501 (m, n−1), 501 (m, n),and 501 (m, n+1) are provided in a stripe arrangement in the columndirection is shown in FIG. 3A, one embodiment of the present inventionis not limited thereto. For example, a stripe arrangement in the rowdirection shown in FIG. 3B may be employed. Alternatively, although notillustrated, delta arrangement or pentile arrangement may be used. FIG.3B is a schematic view illustrating display regions of the pixel circuit501 (m, n) and pixel circuits 501 (m−1, n) and 501 (m+1, n) which areadjacent to the pixel circuit 501 (m, n) in the row direction of thepixel circuit 501 (m, n).

The pixel circuit 501 (m, n), the pixel circuit 501 (m−1, n), and thepixel circuit 501 (m+1, n) illustrated in FIG. 3B each include thedisplay region 430 d functioning as a display region of the displayelement 430 and the display region 630 d functioning as a display regionof the display element 630. The structures of the display regions 430 dand 630 d may be similar to those shown in FIG. 3A.

<1-7. Structure Example of Display Device (Top View)>

Next, a specific structure example of the display device 500 illustratedin FIG. 1 will be described with reference to FIGS. 4A and 4B and FIG.5.

FIG. 4A is a top view of the display device 500. As described above, thedisplay device 500 includes the pixel portion 502, the gate drivercircuit portions 504 a and 504 b and the source driver circuit portion506 placed outside the pixel portion 502. FIG. 4A schematicallyillustrates the pixel circuit 501 (m, n) included in the pixel portion502. A flexible printed circuit (FPC) is electrically connected to thedisplay device 500 in FIG. 4A.

FIG. 4B is a top view schematically illustrating the pixel circuit 501(m, n) shown in FIG. 4A and the pixel circuit 501 (m, n+1) adjacent tothe pixel circuit 501 (m, n). The signal lines S_(L) _(_) _(n), S_(L)_(_) _(n+1), S_(E) _(_) _(n), and S_(E) _(_) _(n+1), the scan linesG_(L) _(_) _(m) and G_(E) _(_) _(m), the common line COM_(m), and thetransistors Tr1, Tr2, and Tr3 in FIG. 4B respectively correspond to thereference numerals in FIG. 2. The display region 430 d and the displayregion 630 d in FIG. 4B correspond to the reference numerals in FIG. 3A.A common line COM_(—m+1) in FIG. 4B indicates a common line included inthe pixel circuit 501 (m, n+1) adjacent to the pixel circuit 501 (m, n).

<1-8. Structure Example of Display Device (Cross Section)>

Next, a cross-sectional structure of the display device 500 will bedescribed with reference to FIG. 5.

FIG. 5 is a cross-sectional view corresponding to cross sections takenalong the dashed-dotted lines A1-A2, A3-A4, A5-A6, A7-A8, A9-A10, andA11-A12 illustrated in FIGS. 4A and 4B.

A cross section taken along the dashed-dotted line A1-A2 corresponds toa region in which the FPC is attached to the display device 500. A crosssection taken along the dashed-dotted line A3-A4 corresponds to a regionin which the gate driver circuit portion 504 a is provided. A crosssection taken along the dashed-dotted line A5-A6 corresponds to a regionin which the display element 430 and the display element 630 areprovided. A cross section taken along the dashed-dotted line A7-A8corresponds to a region in which the display element 430 is provided. Across section taken along the dashed-dotted line A9-A10 corresponds to aconnection region of the display device 500. A cross section taken alongthe dashed-dotted line A11-A12 corresponds to the edge of the displaydevice 500 and the vicinity thereof.

In FIG. 5, the display device 500 includes the display element 430, thedisplay element 630, the transistor Tr1, the transistor Tr3, and atransistor Tr4 between a substrate 452 and a substrate 652.

As described above, the display element 430 has a function of reflectingincident light and the display element 630 has a function of emittinglight. In FIG. 5, the light entering the display element 430 and thereflected light are schematically denoted by arrows of dashed lines.Furthermore, the light emitted from the display element 630 isschematically denoted by an arrow of a dashed double-dotted line.

[Pixel Circuit]

The cross sections taken along the dashed-dotted lines A5-A6 and A7-A8in FIG. 5 are described with reference to FIG. 6. FIG. 6 corresponds toan enlarged cross-sectional view of some components taken along thedashed-dotted lines A5-A6 and A7-A8 in FIG. 5. Further, the enlargedcross-sectional view is reversed upside down. Note that in FIG. 6, somecomponents are not illustrated in order to avoid complexity of thedrawing.

The display element 430 includes a conductive film 403 b, a liquidcrystal layer 620, and a conductive film 608. The conductive film 403 bfunctions as a pixel electrode and the conductive film 608 functions asa counter electrode. The conductive film 403 b is electrically connectedto the transistor Tr1.

The display element 430 includes conductive films 405 b and 405 celectrically connected to the conductive film 403 b. The conductivefilms 405 b and 405 c each have a function of reflecting incident light.That is, the conductive films 405 b and 405 c function as reflectivefilms. An opening 450 transmitting incident light is provided in thereflective films. In FIG. 6, a conductive film functioning as areflective film is separated into island shapes by the opening 450,whereby the conductive film 405 c is positioned below the transistor Tr1and the conductive film 405 b is positioned below the transistor Tr3.Since light of the display element 630 is emitted from the opening 450,the opening 450 corresponds to the display region 630 d illustrated inFIG. 5.

The display element 630 has a function of emitting light toward theopening 450. In FIG. 6, the display element 630 is a bottom emissiontype light-emitting element.

The display element 630 includes a conductive film 417, an EL layer 419,and a conductive film 420. The conductive film 417 functions as a pixelelectrode and an anode electrode. The conductive film 420 functions as acounter electrode and a cathode electrode. Although a description ismade on a structure where the conductive film 417 functions as an anodeelectrode and the conductive film 420 functions as a cathode electrodein this embodiment, one embodiment of the present invention is notlimited thereto. For example, the conductive film 417 may function as acathode electrode and the conductive film 420 may function as an anodeelectrode.

The conductive film 417 is electrically connected to the transistor Tr3.

The transistors Tr1 and Tr3 each include an oxide semiconductor film.The conductive films 403 b and 417 functioning as pixel electrodes eachcontain at least one metal element contained in the oxide semiconductorfilms included in the transistors Tr1 and Tr3.

For example, in the case where oxide semiconductor films are used inchannel regions of the transistors Tr1 and Tr3 and oxide semiconductorfilms having the same composition as the oxide semiconductor films whichare used in the channel regions are used in the conductive films 403 band 417 functioning as pixel electrodes, manufacturing cost can bereduced. As illustrated in FIG. 6, since a plurality of insulatingfilms, conductive films, semiconductor films, or the like are necessaryin a display device including a plurality of display elements and aplurality of transistors, it is important to use the same material indifferent processes.

Each of the transistors Tr1 and Tr3 preferably has a staggered(top-gate) structure as illustrated in FIG. 6. With the staggeredtransistors, parasitic capacitance between the gate electrode and eachof the source electrode and the drain electrode can be reduced.

The transistor Tr1 is formed over an insulating film 406 and aninsulating film 408 and includes an oxide semiconductor film 409 c overthe insulating film 408, an insulating film 410 c over the oxidesemiconductor film 409 c, and an oxide semiconductor film 411 c over theinsulating film 410 c. The insulating film 410 c functions as a gateinsulating film and the oxide semiconductor film 411 c functions as agate electrode.

Insulating films 412 and 413 are provided over the oxide semiconductorfilms 409 c and 411 c. An opening reaching the oxide semiconductor film409 c is provided in the insulating films 412 and 413 and conductivefilms 414 f and 414 g are electrically connected to the oxidesemiconductor film 409 c through the opening. The conductive films 414 fand 414 g function as a source electrode and a drain electrode of thetransistor Tr1.

Insulating films 416 and 418 are provided over the transistor Tr1.

The transistor Tr3 is formed over the insulating film 406 and includes aconductive film 407 b over the insulating film 406, the insulating film408 over the conductive film 407 b, an oxide semiconductor film 409 bover the insulating film 408, an insulating film 410 b over the oxidesemiconductor film 409 b, and an oxide semiconductor film 411 b over theinsulating film 410 b. The conductive film 407 b functions as a firstgate electrode and the insulating film 408 functions as a first gateinsulating film. The insulating film 410 b functions as a second gateinsulating film and the oxide semiconductor film 411 b functions as asecond gate electrode.

Insulating films 412 and 413 are provided over the oxide semiconductorfilms 409 b and 411 b. An opening reaching the oxide semiconductor film409 b is provided in the insulating films 412 and 413 and conductivefilms 414 d and 414 e are electrically connected to the oxidesemiconductor film 409 b through the opening. The conductive films 414 dand 414 e function as a source electrode and a drain electrode of thetransistor Tr3.

A conductive film 414 e is electrically connected to a conductive film407 f through an opening provided in the insulating films 406, 408, 412,and 413. The conductive film 407 f is formed through the same process asthat of the conductive film 407 b and functions as a connectionelectrode.

The insulating film 416 and the conductive film 417 are provided overthe transistor Tr3. An opening reaching the conductive film 414 d isprovided in the insulating film 416, and the conductive film 414 d andthe conductive film 417 are electrically connected to each other throughthe opening.

An insulating film 418, the EL layer 419, and the conductive film 420are provided over the conductive film 417. An opening reaching theconductive film 417 is provided in the insulating film 418, and theconductive film 417 and the EL layer 419 are electrically connected toeach other through the opening.

The conductive film 420 is adhered to the substrate 452 with a sealant454 placed therebetween.

A coloring film 604, an insulating film 606, and the conductive film 608are provided over the substrate 652 which faces the substrate 452. Afunctional film 626 is provided below the substrate 652. Light reflectedby the display element 430 and light emitted from the display element630 are extracted through the coloring film 604, the functional film626, and the like.

The display element 430 includes alignment films 618 a and 618 b incontact with the liquid crystal layer 620 as illustrated in FIG. 6. Notethat a structure without the alignment films 618 a and 618 b may beemployed.

When the structures of the transistors Tr1 and Tr3 are made differentfrom each other as illustrated in FIG. 6, the area of the circuit can bereduced. Specifically, the transistor Tr1 is a single-gate transistor inwhich the oxide semiconductor film 411 c functioning as a gate electrodeis provided, whereas the transistor Tr3 is a multi-gate transistor inwhich the conductive film 407 b functioning as a first gate electrodeand the oxide semiconductor film 411 b functioning as a second gateelectrode are provided. Note that there is no limitation on thestructure of the transistor which is used in the display device of oneembodiment of the present invention. For example, both transistors Tr1and Tr3 may have either a single-gate structure or a multi-gatestructure.

[FPC and Gate Driver Circuit Portion]

The cross-sections taken along the dashed-dotted line A1-A2 and thedashed-dotted line A3-A4 in FIG. 5 are described with reference to FIG.7. FIG. 7 corresponds to an enlarged cross-sectional view of componentstaken along the dashed-dotted lines A1-A2 and A3-A4 in FIG. 5. Further,the enlarged cross-sectional view is reversed upside down. Note that inFIG. 7, some components are not illustrated in order to avoid complexityof the drawing.

The FPC illustrated in FIG. 7 is electrically connected to a conductivefilm 403 a through an anisotropic conductive film (ACF). An insulatingfilm 404 is provided over the conductive film 403 a. An opening reachingthe conductive film 403 a is provided in the insulating film 404, andthe conductive film 403 a and a conductive film 405 a are electricallyconnected to each other through the opening.

The insulating film 406 is provided over the conductive film 405 a. Anopening reaching the conductive film 405 a is provided in the insulatingfilm 406, and the conductive film 405 a and a conductive film 407 a areelectrically connected to each other through the opening. The insulatingfilms 408, 412, and 413 are provided over the conductive film 407 a. Anopening reaching the conductive film 407 a is provided in the insulatingfilms 408, 412, and 413 and the conductive film 407 a and a conductivefilm 414 a are electrically connected to each other through the opening.

The insulating films 416 and 418 are provided over the insulating film413 and the conductive film 414 a. The insulating film 418 is adhered tothe substrate 452 with the sealant 454 placed therebetween.

The transistor Tr4 illustrated in FIG. 7 corresponds to a transistorincluded in the gate driver circuit portion 504 a.

The transistor Tr4 is formed over the insulating film 406 and includes aconductive film 407 e over the insulating film 406, the insulating film408 over the conductive film 407 e, an oxide semiconductor film 409 aover the insulating film 408, an insulating film 410 a over the oxidesemiconductor film 409 a, and an oxide semiconductor film 411 a over theinsulating film 410 a. The conductive film 407 e functions as a firstgate electrode. The insulating film 410 a functions as a second gateinsulating film and the oxide semiconductor film 411 a functions as asecond gate electrode.

The insulating films 412 and 413 are provided over the oxidesemiconductor films 409 a and 411 a. An opening reaching the oxidesemiconductor film 409 a is provided in the insulating films 412 and 413and conductive films 414 b and 414 c are electrically connected to theoxide semiconductor film 409 a through the opening. The conductive films414 b and 414 c function as a source electrode and a drain electrode ofthe transistor Tr4.

The transistor Tr4 is a multi-gate transistor like the transistor Tr3which is described above. A multi-gate transistor is preferably used inthe gate driver circuit portion 504 a because the current drivecapability can be improved. Since the use of a multi-gate transistor canimprove the current drive capability, the width of the driver circuitcan be reduced.

The insulating films 416 and 418 are provided over the transistor Tr4.The insulating film 418 is adhered to the substrate 452 with the sealant454 placed therebetween.

A light-blocking film 602, the insulating film 606, and the conductivefilm 608 are provided over the substrate 652 which faces the substrate452.

A structure body 610 a is formed in a position overlapping with thetransistor Tr4 over the conductive film 608. The structure body 610 ahas a function of controlling the thickness of the liquid crystal layer620. The alignment films 618 a and 618 b are formed between thestructure body 610 a and the insulating film 404 in FIG. 7. Note thatthe alignment films 618 a and 618 b are not necessarily formed betweenthe structure body 610 a and the insulating film 404.

A sealant 622 is provided at an end portion of the substrate 652. Notethat the sealant 622 is provided between the substrate 652 and theconductive film 403 a.

[Connection Region and Region In the Vicinity of End Portion]

The cross sections taken along the dashed-dotted line A9-A10 and thedashed-dotted line A11-A12 in FIG. 5 are described with reference toFIG. 8. FIG. 8 corresponds to an enlarged cross-sectional view ofcomponents taken along the dashed-dotted lines A9-A10 and A11-A12 inFIG. 5. Further, the enlarged cross-sectional view is reversed upsidedown. Note that in FIG. 8, some components are not illustrated in orderto avoid complexity of the drawing.

In FIG. 8, the conductive film 608 is electrically connected to aconductive film 403 c via a conductor 624. The conductor 624 is includedin the sealant 622. The conductive film 608 is provided over thesubstrate 652, the light-blocking film 602, and the insulating film 606.

The insulating film 404 is provided over the conductive film 403 c. Anopening reaching the conductive film 403 c is provided in the insulatingfilm 404, and the conductive film 403 c and a conductive film 405 d areelectrically connected to each other through the opening. The insulatingfilm 406 is provided over the conductive film 405 d. An opening reachingthe conductive film 405 d is provided in the insulating film 406, andthe conductive film 405 d and a conductive film 407 d are electricallyconnected to each other through the opening.

The insulating films 408, 412, and 413 are provided over the conductivefilm 407 d. An opening reaching the conductive film 407 d is provided inthe insulating films 408, 412, and 413 and the conductive film 407 d anda conductive film 414 h are electrically connected to each other throughthe opening. The insulating films 416 and 418 are provided over theconductive film 414 h. The insulating film 418 is adhered to thesubstrate 452 with the sealant 454 placed therebetween.

The sealant 622 is provided at end portions of the substrate 452 and652. Note that the sealant 622 is provided between the substrate 652 andthe insulating film 404.

<1-9. Manufacturing Method of Display Device>

Next, a method for manufacturing the display device 500 illustrated inFIG. 5 is described with reference to FIGS. 9A to 9C, FIGS. 10A to 10C,FIGS. 11A to 11C, FIGS. 12A to 12C, FIGS. 13A and 13B, and FIG. 14.FIGS. 9A to 9C, FIGS. 10A to 10C, FIGS. 11A to 11C, FIGS. 12A to 12C,FIGS. 13A and 13B, and FIG. 14 are cross-sectional views illustrating amethod for manufacturing the display device 500.

First, a conductive film 402 is formed over a substrate 401. Then, aconductive film is formed over the conductive film 402 and processedinto island shapes, whereby the conductive films 403 a, 403 b, and 403 care formed (see FIG. 9A).

The conductive film 402 has a function of a separation layer, theconductive films 403 a and 403 c each have a function of a connectionelectrode, and the conductive film 403 b has a function of a pixelelectrode. In this embodiment, a tungsten film is used as the conductivefilm 402 and an In—Ga—Zn oxide is used as the conductive films 403 a,403 b, and 403 c. As the In—Ga—Zn oxide, an IGZO film (In:Ga:Zn=4:2:4.1[atomic ratio]) is used.

An insulating film is formed over the conductive films 402, 403 a, 403b, and 403 c and openings are formed in desired regions of theinsulating film, whereby the insulating film 404 is formed. Then, aconductive film is formed over the conductive films 403 a, 403 b, and403 c and the insulating film 404 and processed into island shapes,whereby the conductive films 405 a, 405 b, 405 c, and 405 d are formed(see FIG. 9B).

The insulating film 404 has openings in regions overlapping with theconductive films 403 a, 403 b, and 403 c. The conductive film 403 a iselectrically connected to the conductive film 405 a through the opening,the conductive film 403 b is electrically connected to the conductivefilms 405 b and 405 c through the openings, and the conductive film 403c is electrically connected to the conductive film 405 d through theopening. In this embodiment, a silicon oxynitride film is used as theinsulating film 404 and alloy films of silver, palladium, and copper areused as the conductive films 405 a, 405 b, 405 c, and 405 d. Thus, metalfilms with high reflectivity (e.g., a film containing silver) arepreferably used as the conductive films 405 a, 405 b, 405 c, and 405 dso that they can function as reflective films.

An insulating film is formed over the insulating film 404 and theconductive films 405 a, 405 b, 405 c, and 405 d and openings are formedin desired regions of the insulating film, whereby the insulating film406 is formed. A conductive film is formed over the conductive films 405a, 405 b, 405 c, and 405 d and the insulating film 406 and processedinto island shapes, whereby the conductive films 407 a, 407 b, 407 c,407 d, 407 e, 407 f, and 407 g are formed (see FIG. 9C).

The insulating film 406 has openings in regions overlapping with theconductive films 405 a, 405 c, and 405 d. Through the openings, theconductive film 405 a, the conductive film 405 c, and the conductivefilm 405 d are electrically connected to the conductive film 407 a, theconductive film 407 c, and the conductive film 407 d, respectively. Inthis embodiment, a silicon oxynitride film is used as the insulatingfilm 406, and a stack of a tantalum nitride film and a copper film isused as each of the conductive films 407 a, 407 b, 407 c, 407 d, 407 e,407 f, and 407 g.

Next, the insulating film 408 is formed over the insulating film 406 andthe conductive films 407 a, 407 b, 407 c, 407 d, 407 e, 407 f, and 407g. Then, an oxide semiconductor film is formed over the insulating film408 and processed into island shapes, whereby the oxide semiconductorfilms 409 a, 409 b, and 409 c are formed (see FIG. 10A).

In this embodiment, a silicon oxynitride film is used as the insulatingfilm 408 and an In—Ga—Zn oxide is used for the oxide semiconductor films409 a, 409 b, and 409 c. The In—Ga—Zn oxide preferably has the samecomposition as the oxide semiconductor film used as the conductive films403 a, 403 b, and 403 c. When the oxide semiconductor film having thesame composition is used as the oxide semiconductor films 409 a, 409 b,and 409 c and the conductive films 403 a, 403 b, and 403 c, themanufacturing cost can be reduced.

Next, an insulating film and an oxide semiconductor film are formed overthe insulating film 408 and the oxide semiconductor films 409 a, 409 b,and 409 c and processed into desired shapes, whereby the island-shapedinsulating films 410 a, 410 b, and 410 c and the island-shaped oxidesemiconductor films 411 a, 411 b, and 411 c are formed (see FIG. 10B).

In this embodiment, a silicon oxynitride film is used as the insulatingfilms 410 a, 410 b, and 410 c and an In—Ga—Zn oxide is used as oxidesemiconductor films 411 a, 411 b, and 411 c. The In—Ga—Zn oxidepreferably has the same composition as the oxide semiconductor film usedas the conductive films 403 a, 403 b, and 403 c and the oxidesemiconductor films 409 a, 409 b, and 409 c. With use of the oxidesemiconductor film having the same composition as the oxidesemiconductor films 411 a, 411 b, and 411 c, and oxide semiconductorfilms 409 a, 409 b, and 409 c and the conductive films 403 a, 403 b, and403 c, the manufacturing cost can be reduced.

Next, insulating films are formed over the insulating film 408 and theoxide semiconductor films 409 a, 409 b, and 409 c and openings areformed in desired regions of the insulating films, whereby theinsulating films 412 and 413 are formed (see FIG. 10C).

Although a stacked-layer structure of two layers of the insulating films412 and 413 is illustrated in FIG. 10C, the present invention is notlimited thereto. For example, a single-layer structure of the insulatingfilm 412, a single-layer structure of the insulating film 413, or astacked-layer structure of three or more layers in which the insulatingfilms 412 and 413 and another insulating film are stacked may be used.In this embodiment, a silicon nitride film is used as the insulatingfilm 412 and a silicon nitride oxide film is used as the insulating film413.

Openings are formed in part of the insulating film 408 when openings areformed in the insulating films 412 and 413. Openings formed in theinsulating films 408, 412, and 413 reach the conductive films 407 a, 407c, 407 d, and 407 f.

Next, a conductive film is formed over the insulating film 413 andprocessed into desired shapes, whereby the conductive films 414 a, 414b, 414 c, 414 d, 414 e, 414 f, 414 g, and 414 h are formed (see FIG.11A).

The conductive films 414 b and 414 c function as a source electrode anda drain electrode of the transistor Tr4. The conductive films 414 d and414 e function as a source electrode and a drain electrode of thetransistor Tr3. The conductive films 414 f and 414 g function as asource electrode and a drain electrode of the transistor Tr1.

In the transistor Tr1, the conductive film 414 g is electricallyconnected to the conductive film 403 b with the conductive films 407 cand 405 c placed therebetween. The transistor Tr1 can control thepotential of the conductive film 403 b.

In this embodiment, a stack of tantalum nitride and copper is preferablyused as each of the conductive films 414 a, 414 b, 414 c, 414 d, 414 e,414 f, 414 g, and 414 h. The conductive films 407 a, 407 b, 407 c, 407d, 407 e, 407 f, and 407 g are preferably formed using the same materialas the conductive films 414 a, 414 b, 414 c, 414 d, 414 e, 414 f, 414 g,and 414 h because the manufacturing cost can be reduced. In thestructure where the conductive film contains copper, a signal delay orthe like can be suppressed even when a large substrate (e.g., an 8thgeneration mother glass (2160 mm×2460 mm), a 9th generation mother glass(2400 mm×2800 mm or 2450 mm×3050 mm), or a 10 th generation mother glass(2950 mm×3400 mm)) is used.

Next, the insulating film 416 is formed so as to cover the transistorsTr1, Tr3, and Tr4. The insulating film 416 has an opening in a regionoverlapping with the conductive film 414 d. Next, a conductive film isformed over the insulating film 416 and the conductive film 414 d andprocessed into a desired shape, whereby the conductive film 417 isformed. Then, the insulating film 418 is formed in a desired region overthe insulating film 416 and the conductive film 417 (see FIG. 11B).

The insulating film 418 has an opening in a region overlapping with theconductive film 417. In this embodiment, an acrylic resin film is usedas the insulating film 416, an In—Sn—Si oxide (also referred to as ITSO)is used as the conductive film 417, and a polyimide resin film is usedas the insulating film 418.

Next, the EL layer 419 is formed over the conductive film 417 and theinsulating film 418, and the conductive film 420 is formed over the ELlayer 419 (see FIG. 11C).

The display element 630 is formed of the conductive film 417, the ELlayer 419, and the conductive film 420. Note that the conductive film417 functions as one of the pair of electrodes of the display element630 and the conductive film 420 functions as the other of the pair ofelectrodes of the display element 630. Although not illustrated, the ELlayer 419 is formed differently based on color elements (RGB). In thisembodiment, a phosphorescent material is used for light-emitting layersof R and G, and a fluorescent material is used for a light-emittinglayer of B. In this embodiment, an alloy film of silver and magnesium isused as the conductive film 420.

Through the above steps, an element formed over the substrate 401 can befabricated.

A method for manufacturing the substrate 652 placed to face thesubstrate 452 is described with reference to FIGS. 12A to 12C.

First, the light-blocking film 602 is formed over the substrate 652.After that, the coloring film 604 is formed over the substrate 652 andthe light-blocking film 602 (see FIG. 12A).

In this embodiment, a titanium film is used as the light-blocking film602 and an acrylic resin film containing pigment is used as the coloringfilm 604.

Next, the insulating film 606 is formed over the light-blocking film 602and the coloring film 604. Then, the conductive film 608 is formed overthe insulating film 606 (see FIG. 12B).

In this embodiment, an acrylic resin film is used as the insulating film606 and an ITSO film is used as the conductive film 608.

Next, the structure bodies 610 a and 610 b are formed in desired regionsover the conductive film 608. Then, the alignment film 618 b is formedover the conductive film 608 and the structure bodies 610 a and 610 b(see FIG. 12C).

Note that a structure without the alignment film 618 b may be employed.In this embodiment, acrylic resin films are used as the structure bodies610 a and 610 b and a polyimide resin film is used as the alignment film618 b. Although the structure bodies 610 a and 610 b are formed over thesubstrate 652 in this embodiment, the present invention is not limitedthereto. For example, the structure bodies 610 a and 610 b may be formedover the element which is formed over the substrate 401 described above.

Through the above steps, an element formed over the substrate 652 can befabricated.

Next, the element formed over the substrate 401 is separated from thesubstrate 401. Specifically, separation is conducted at an interfacebetween the conductive film 402 formed over the substrate 401 and theconductive films 403 a, 403 b, and 403 c and the insulating film 404which are formed over the conductive film 402. For the separation, thesealant 454 is formed over the element which is formed over thesubstrate 401. Then, the substrate 452 is attached to the sealant 454and the element is separated from the interface between the element andthe conductive film 402 (see FIG. 13A).

When the element is separated from the interface between the element andthe conductive film 402, surfaces of the conductive films 403 a, 403 b,and 403 c (rear surfaces of the conductive films 403 a, 403 b, and 403 cin FIG. 13A) are exposed. In the case where an insulating film, aforeign substance, or the like is attached to the surfaces of theconductive films 403 a, 403 b, and 403 c, the insulating film, theforeign substance, or the like is preferably removed by cleaningtreatment, ashing treatment, etching treatment, or the like.

When the element is separated from the interface between the element andthe conductive film 402, a polar solvent (typically water), a nonpolarsolvent, or the like is preferably added to the interface between theconductive film 402 and the conductive films 403 a, 403 b, and 403 c andthe insulating film 404 which are formed over the conductive film 402.For example, it is preferable to use water in separating the elementfrom the interface between the element and the conductive film 402because damage caused by electrification in separation can be reduced.

As the conductive film 402, any of the following materials can be used.The conductive film 402 can have a single-layer structure or astacked-layer structure containing an element selected from tungsten,molybdenum, titanium, tantalum, niobium, nickel, cobalt, zirconium,zinc, ruthenium, rhodium, palladium, osmium, iridium, and silicon; analloy material containing any of the elements; or a compound materialcontaining any of the elements. In the case of a layer containingsilicon, a crystal structure of the layer containing silicon may beamorphous, microcrystal, polycrystal, or single crystal.

When the conductive film 402 is formed as a stacked layer structureincluding a layer which includes tungsten and a layer which includes anoxide of tungsten, the layer which includes tungsten may be formed andan insulating layer which includes an oxide may be formed thereover sothat the layer which includes an oxide of tungsten is formed at theinterface of the tungsten layer and the insulating layer. Alternatively,the layer containing an oxide of tungsten may be formed by performingthermal oxidation treatment, oxygen plasma treatment, dinitrogenmonoxide (N₂O) plasma treatment, treatment with a highly oxidizingsolution such as ozone water, or the like on the surface of the layercontaining tungsten. Plasma treatment or heat treatment may be performedin an atmosphere of oxygen, nitrogen, or dinitrogen monoxide alone, or amixed gas of any of these gasses and another gas. The surface conditionof the conductive film 402 is changed by the plasma treatment or theheat treatment, whereby adhesion between the conductive film 402 and theconductive films 403 a, 403 b, and 403 c and the insulating film 404which are formed later can be controlled.

Although the structure where the conductive film 402 is provided isdescribed in this embodiment, the present invention is not limitedthereto. Note that a structure where the conductive film 402 is notprovided may be employed. In that case, an organic resin film may beformed in a region in which the conductive film 402 is formed. As theorganic resin film, for example, a polyimide resin film, a polyamideresin film, an acrylic resin film, an epoxy resin film, or a phenolresin film can be used.

In the case where the organic resin film is used instead of theconductive film 402, as a method for separating the element formed overthe substrate 401, a laser light is irradiated from the lower side ofthe substrate 401 to weaken the organic resin film, whereby separationis conducted at an interface between the substrate 401 and the organicresin film or the interface between the organic resin film and theconductive films 403 a, 403 b, and 403 c and the insulating film 404.

In the case where the laser light is irradiated, a region having strongadhesion and a region having weak adhesion are formed between thesubstrate 401 and the conductive films 403 a, 403 b, and 403 c and theinsulating film 404 by adjustment of the irradiation energy density ofthe laser light. After the region having strong adhesion and the regionhaving weak adhesion are formed, only the region having weak adhesionmay be separated.

Next, the element is reversed so that the substrate 452 is placed at thebottom, and the alignment film 618 a is formed over the insulating film404 and the conductive film 403 b (see FIG. 13B).

For the alignment film 618 a, a material similar to that of thealignment film 618 b may be used.

Next, an element over the substrate 452 and an element over thesubstrate 652 are attached to each other and sealed with the sealant622. After that, the liquid crystal layer 620 is formed between thesubstrates 452 and 652, whereby the display element 430 is formed (seeFIG. 14).

Note that the conductor 624 is provided in the sealant 622 over theconductive film 403 c. As the conductor 624, conductive particles may beprovidedinto a desired region in the sealant 622 using a dispensermethod or the like. The conductive film 403 c and the conductive film608 are electrically connected to each other via the conductor 624.

Next, the functional film 626 is formed over the substrate 652 (see FIG.14).

Note that the functional film 626 is not necessarily formed.

After that, the FPC is bonded to the conductive film 403 a with the ACFplaced therebetween. Note that an anisotropic conductive paste (ACP) maybe used instead of the ACF.

Through the above steps, the display device 500 illustrated in FIG. 5can be manufactured.

<1-10. Modification Example 1 of Display Device>

A touch panel may be provided in the display device 500 illustrated inFIG. 5. As the touch panel, a capacitive touch panel (a surfacecapacitive touch panel or a projected capacitive touch panel) can bepreferably used.

A structure in which a touch panel is provided in the display device 500is described with reference to FIGS. 15, 16, and 17.

FIG. 15 is a cross-sectional view of a structure in which a touch panel691 is provided in the display device 500. FIG. 16 is a cross-sectionalview of a structure in which a touch panel 692 is provided in thedisplay device 500. FIG. 17 is a cross-sectional view of a structure inwhich a touch panel 693 is provided in the display device 500.

The touch panel 691 illustrated in FIG. 15 is a so-called in-cell touchpanel which is provided between the substrate 652 and the coloring film604. The touch panel 691 is formed over the substrate 652 before thelight-blocking film 602 and the coloring film 604 are formed.

The touch panel 691 includes a light-blocking film 662, an insulatingfilm 663, a conductive film 664, a conductive film 665, an insulatingfilm 666, a conductive film 667, and an insulating film 668. Changes inthe mutual capacitance in the conductive films 664 and 665 can bedetected when an object such as a finger or a stylus approaches, forexample.

An intersection portion of the conductive film 664 and the conductivefilm 665 is shown above the transistor Tr4 illustrated in FIG. 15. Theconductive film 667 is electrically connected to the two conductivefilms 664 between which the conductive film 665 is sandwiched throughopenings provided in the insulating film 666. Although a region in whichthe conductive film 667 is provided is located in a region correspondingto the gate driver circuit portion 504 a in FIG. 15, it is not limitedthereto, and the region in which the conductive film 667 is provided maybe provided in a region in which the pixel circuit 501 (m, n) isprovided, for example.

The conductive films 664 and 665 are provided in a region overlappingwith the light-blocking film 662. As illustrated in FIG. 15, it ispreferable that the conductive film 664 do not overlap with the displayelement 630. In other words, the conductive film 664 has openings inregions overlapping with the display element 630. That is, theconductive film 664 has a mesh shape. With this structure, theconductive film 664 does not block light emitted from the displayelement 630. Therefore, since luminance is hardly reduced even when thetouch panel 691 is provided, a display device with high visibility andlow power consumption can be obtained. Note that the conductive film 665can have a structure similar to that of the conductive film 664.

Since the conductive films 664 and 665 do not overlap with the displayelement 630, a metal material whose transmittance of visible light islow can be used for the conductive films 664 and 665. Therefore, ascompared to the case of using an oxide material whose transmittance ofvisible light is high, resistance of the conductive films 664 and 665can be reduced, whereby sensitivity of the sensor of the touch panel canbe increased.

Note that a material that can be used for the light-blocking film 602described later can be used for the light-blocking film 662. For theinsulating films 663, 666, and 668, a material that can be used for theinsulating films 404, 406, 408, 410 a, 410 b, 410 c, 412, 413, 416, 418,and 606 described later can be used. For the conductive films 664, 665,and 667, a material that can be used for the conductive films 402, 403a, 403 b, 403 c, 405 a, 405 b, 405 c, 405 d, 407 a, 407 b, 407 c, 407 d,407 e, 414 a, 414 b, 414 c, 414 d, 414 e, 414 f, 414 g, 414 h, 417, 420,and 608 and the oxide semiconductor films 411 a, 411 b, and 411 cdescribed later can be used.

Conductive nanowires may be used for the conductive films 664, 665, and667. The nanowires may have a mean diameter of greater than or equal to1 nm and less than or equal to 100 nm, preferably greater than or equalto 5 nm and less than or equal to 50 nm, further preferably greater thanor equal to 5 nm and less than or equal to 25 nm. As the nanowire, acarbon nanotube or a metal nanowire such as an Ag nanowire, a Cunanowire, or an Al nanowire may be used. For example, in the case ofusing an Ag nanowire for one or all of the conductive films 664, 665,and 667, a visible light transmittance of 89% or more and a sheetresistance of 40 Ω/square or more and 100 Ω/square or less can beachieved.

The touch panel 692 illustrated in FIG. 16 is a so-called on-cell touchpanel which is provided above the substrate 652. The touch panel 692 hasa similar structure to that of the touch panel 691.

The touch panel 693 illustrated in FIG. 17 is provided over a substrate672 and is bonded to the substrate 652 with an adhesive agent 674. Thetouch panel 693 is a so-called out-cell touch panel (also referred to asan externally attached touch panel). The touch panel 693 has a structuresimilar to that of the touch panel 691. In this manner, the displaydevice of one embodiment of the present invention can be combined withvarious types of touch panels.

<1-11. Modification Example 2 of Display Device>

An example of a structure where the liquid crystal element of thedisplay device 500 illustrated in FIG. 5 is a horizontal electric fieldmode liquid crystal element, (here, an FFS mode liquid crystal element)is shown in FIG. 18.

The display device 500 illustrated in FIG. 18 includes an insulatingfilm 681 over the conductive films 403 b and 403 c and a conductive film682 over the insulating film 681 in addition to the above-mentionedcomponents.

The insulating film 681 has an opening in a connection region takenalong the dashed-dotted line A9-A10, and the conductive film 682 iselectrically connected to the conductive film 403 c through the opening.In FIG. 18, the conductor 624 included in the sealant 622 in FIG. 14 isnot provided.

The conductive film 682 functions as a common electrode. The conductivefilm 682 may have a comb-like shape or a shape having a slit when seenfrom the above. Since the conductive film 682 is provided in the displaydevice 500 illustrated in FIG. 18, the conductive film 608 provided onthe substrate 652 side in FIG. 14 is not provided. Note that theconductive film 682 may be provided and the conductive film 608 may befurther provided on the substrate 652 side.

For the insulating film 681, a material that can be used for theinsulating films 404, 406, 408, 410 a, 410 b, 410 c, 412, 413, 416, 418,and 606 described later can be used. For the conductive film 682, amaterial that can be used for the conductive film 402, 403 a, 403 b, 403c, 405 a, 405 b, 405 c, 405 d, 407 a, 407 b, 407 c, 407 d, 407 e, 411 a,411 b, 411 c, 414 d, 414 e, 414 f, 414 g, 414 h, 417, 420, and 608 andthe oxide semiconductor films 411 a, 411 b, and 411 c described latercan be used.

When the conductive film 682 is formed using a light-transmittingmaterial, a light-transmitting capacitor can be formed. Thelight-transmitting capacitor includes the conductive film 682, theinsulating film 681 overlapping with the conductive film 682, and theconductive film 403 c. This structure is preferable because the amountof charge accumulated in the capacitor can be increased.

<1-12. Components of Display Device>

Next, the components of the display device 500 and the manufacturingmethod thereof illustrated in FIG. 5 to FIG. 14 are described below.

[Substrate]

The substrates 401, 452, and 652 can be formed using a material havingheat resistance high enough to withstand heat treatment in themanufacturing process.

Specifically, non-alkali glass, soda-lime glass, potash glass, crystalglass, quartz, sapphire, or the like can be used. Alternatively, aninorganic insulating film may be used. Examples of the inorganicinsulating film include a silicon oxide film, a silicon nitride film, asilicon oxynitride film, and an alumina film.

The non-alkali glass preferably has a thickness of greater than or equalto 0.2 nm and less than or equal to 0.7 mm, for example. The non-alkaliglass may be polished to obtain the above thickness.

For example, a large-sized glass substrate having any of the followingsizes can be used as each of the substrates 401, 452, and 652: the 6thgeneration (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm),and the 10 th generation (2950 mm×3400 mm). Thus, a large-sized displaydevice can be manufactured.

Alternatively, as the substrates 401, 452, and 652, a single crystalsemiconductor substrate or a polycrystalline semiconductor substratemade of silicon or silicon carbide, a compound semiconductor substratemade of silicon germanium or the like, an SOI substrate, or the like maybe used.

Alternatively, for the substrates 401, 452, and 652, an inorganicmaterial such as a metal may be used. Examples of the inorganic materialsuch as a metal include stainless steel or aluminum.

Alternatively, for the substrates 401, 452, and 652, an organic materialsuch as a resin, a resin film, or plastic may be used. Examples of theresin film include polyester, polyolefin, polyamide (e.g., nylon oraramid), polyimide, polycarbonate, polyurethane, an acrylic resin, anepoxy resin, polyethylene terephthalate (PET), polyethylene naphthalate(PEN), polyether sulfone (PES), and a resin having a siloxane bond.

Alternatively, for the substrates 401, 452, and 652, a compositematerial of a combination of an inorganic material and an organicmaterial may be used. Examples of the composite material include a resinfilm to which a metal plate or a thin glass plate is bonded, a resinfilm into which a fibrous or particulate metal or a fibrous orparticulate glass is dispersed, and an inorganic material into which afibrous or particulate resin is dispersed.

[Conductive Film]

A metal film having conductivity, a conductive film having a function ofreflecting visible light, or a conductive film having a function oftransmitting visible light may be used as the conductive films 402, 403a, 403 b, 403 c, 405 a, 405 b, 405 c, 405 d, 407 a, 407 b, 407 c, 407 d,407 e, 414 a, 414 b, 414 c, 414 d, 414 e, 414 f, 414 g, 414 h, 417, 420,and 608 and the oxide semiconductor films 411 a, 411 b, and 411 c.

A material containing a metal element selected from aluminum, gold,platinum, silver, copper, chromium, tantalum, titanium, molybdenum,tungsten, nickel, iron, cobalt, palladium, and manganese can be used forthe metal film having conductivity. Alternatively, an alloy containingany of the above metal elements may be used.

For the metal film having conductivity, specifically a two-layerstructure in which a copper film is stacked over a titanium film, atwo-layer structure in which a copper film is stacked over a titaniumnitride film, a two-layer structure in which a copper film is stackedover a tantalum nitride film, or a three-layer structure in which atitanium film, a copper film, and a titanium film are stacked in thisorder may be used. In particular, a conductive film containing a copperelement is preferably used because the resistance can be reduced. As anexample of the conductive film containing a copper element, an alloyfilm containing copper and manganese is given. The alloy film ispreferable because it can be processed by a wet etching method.

As the metal film having conductivity, a conductive macromolecule or aconductive polymer may be used.

For the conductive film having a function of reflecting visible light, amaterial containing a metal element selected from gold, silver, copper,and palladium can be used. In particular, a conductive film containing asilver element is preferably used because reflectance of visible lightcan be improved.

For the conductive film having a function of transmitting visible light,a material containing an element selected from indium, tin, zinc,gallium, and silicon can be used. Specifically, an In oxide, a Zn oxide,an In—Sn oxide (also referred to as ITO), an In—Sn—Si oxide (alsoreferred to as ITSO), an In—Zn oxide, an In—Ga—Zn oxide, or the like canbe used.

As the conductive film having a function of transmitting visible light,a film containing graphene or graphite may be used. The film containinggraphene can be formed in the following manner: a film containinggraphene oxide is formed and is reduced. As a reducing method, a methodwith application of heat, a method using a reducing agent, or the likecan be employed.

Note that the conductive films 403 c and 417 each having a function of apixel electrode contain at least one metal element contained in theoxide semiconductor films 409 a, 409 b, and 409 c. For example, in thecase where the oxide semiconductor films 409 a, 409 b, and 409 c includea metal oxide such as an In—M—Zn oxide (M is Al, Ga, Y, or Sn), theconductive film 403 c and the conductive film 417 each contain any oneof In, M (M is Al, Ga, Y, or Sn), and Zn.

[Insulating Film]

For the insulating films 404, 406, 408, 410 a, 410 b, 410 c, 412, 413,416, 418, and 606, an inorganic insulating material, an organicinsulating material, or an insulating composite material including aninsulating inorganic material and an insulating organic material can beused.

Examples of the insulating inorganic material include a silicon oxidefilm, a silicon nitride film, a silicon oxynitride film, a siliconnitride oxide film, and an aluminum oxide film. Alternatively, aplurality of the above inorganic materials may be stacked.

As the above insulating organic material, for example, materials thatinclude polyester, polyolefin, polyamide (e.g., nylon or aramid),polyimide, polycarbonate, polyurethane, an acrylic resin, an epoxyresin, or a resin having a siloxane bond can be used. As the insulatingorganic material, a photosensitive material may be used.

[Oxide Semiconductor Film]

The oxide semiconductor films 409 a, 409 b, and 409 c are formed using ametal oxide such as an In—M—Zn oxide (M is Al, Ga, Y, or Sn).Alternatively, an In—Ga oxide or an In—Zn oxide may be used for theoxide semiconductor films 409 a, 409 b, and 409 c.

In the case where the oxide semiconductor films 409 a, 409 b, and 409 cinclude an In—M—Zn oxide, the proportions of In and M, the summation ofwhich is assumed to be 100 atomic %, are as follows: the proportion ofIn is higher than 25 atomic % and the proportion of M is lower than 75atomic %, or the proportion of In is higher than 34 atomic % and theproportion of M is lower than 66 atomic %.

The energy gap of the oxide semiconductor films 409 a, 409 b, and 409 cis preferably 2 eV or more, 2.5 eV or more, or 3 eV or more.

The thickness of the oxide semiconductor films 409 a, 409 b, and 409 cis greater than or equal to 3 nm and less than or equal to 200 nm,preferably greater than or equal to 3 nm and less than or equal to 100nm, further preferably greater than or equal to 3 nm and less than orequal to 60 nm.

In the case where the oxide semiconductor films 409 a, 409 b, and 409 cinclude an In—M—Zn oxide, the atomic ratio of metal elements in asputtering target used for depositing the In—M—Zn oxide preferablysatisfies In≧M and Zn≧M. As the atomic ratio of metal elements in such asputtering target, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=2:1:1.5,In:M:Zn=2:1:2.3, In:M:Zn=2:1:3, In:M:Zn=3:1:2, In:M:Zn=4:2:4.1,In:M:Zn=5:1:7, or the like is preferable. Note that the atomic ratio ofmetal elements in the deposited oxide semiconductor film 108 may varyfrom the above atomic ratio of metal elements in the sputtering targetwithin a range of approximately ±40%. For example, when a sputteringtarget whose atomic ratio of In to Ga and Zn is 4:2:4.1 is used, theatomic ratio of In to Ga and Zn in the deposited oxide semiconductorfilm may be approximately 4:2:3. In the case where a sputtering targetwhose atomic ratio of In to Ga and Zn is 5:1:7 is used, the atomic ratioof In to Ga and Zn in the deposited oxide semiconductor film may beapproximately 5:1:6.

When contained in the oxide semiconductor films 409 a, 409 b, and 409 c,silicon or carbon, which are elements belonging to Group 14, may causeoxygen vacancies to be increased and the oxide semiconductor film tohave n-type conductivity. Thus, the concentration of silicon or carbonin the oxide semiconductor film 108, particularly in the channel region108 i, is set to be lower than or equal to 2×10¹⁸ atoms/cm³ or lowerthan or equal to 2×10¹⁷ atoms/cm³. As a result, the transistor has apositive threshold voltage (normally-off characteristics). Note that theconcentration of silicon or carbon can be measured by secondary ion massspectrometry (SIMS), for example.

Furthermore, the concentration of alkali metal or alkaline earth metalin the oxide semiconductor films 409 a, 409 b, and 409 c, which ismeasured by SIMS, can be lower than or equal to 1×10¹⁸ atoms/cm³ orlower than or equal to 2×10¹⁶ atoms/cm³. Alkali metal and alkaline earthmetal might generate carriers when bonded to an oxide semiconductor, inwhich case the off-state current of the transistor might be increased.Therefore, it is preferable to reduce the concentration of alkali metalor alkaline earth metal in the oxide semiconductor films 409 a, 409 b,and 409 c. As a result, the transistor has a positive threshold voltage(normally-off characteristics).

Furthermore, when contained in the oxide semiconductor films 409 a, 409b, and 409 c, nitrogen may generate electrons serving as carriers andcause carrier density to be increased and the oxide semiconductor films409 a, 409 b, and 409 c to have n-type conductivity. Thus, a transistorincluding an oxide semiconductor film which contains nitrogen is likelyto have normally-on characteristics. For this reason, nitrogen in theoxide semiconductor films 409 a, 409 b, and 409 c is preferably reducedas much as possible. For example, the nitrogen concentration measured bySIMS may be 5×10¹⁸ atoms/cm³ or lower.

When the impurity elements in the oxide semiconductor films 409 a, 409b, and 409 c are reduced, the carrier density of the oxide semiconductorfilms can be lowered. Therefore, the oxide semiconductor films 409 a,409 b, and 409 c can have a carrier density less than or equal to 1×10¹⁷cm⁻³, less than or equal to 1×10¹⁵ cm⁻³, less than or equal to 1×10¹³cm⁻³, or less than or equal to 1×10¹¹ cm⁻³.

When an oxide semiconductor film with a low impurity concentration and alow density of defect states is used as the oxide semiconductor films409 a, 409 b, and 409 c, the transistor can have more excellentelectrical characteristics. Here, the state in which the impurityconcentration is low and the density of defect states is low (the numberof oxygen vacancies is small) is referred to as “highly purifiedintrinsic”, “substantially highly purified intrinsic”, “intrinsic”, or“substantially intrinsic”. A highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor has few carrier generationsources and thus can have a low carrier density in some cases. Thus, atransistor whose channel region is formed in the oxide semiconductorfilm is likely to have a positive threshold voltage (normally-offcharacteristics). The highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film has a low density of defectstates and accordingly has a low density of trap states in some cases.Furthermore, the highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor film enables extremely lowoff-state current. Thus, the transistor whose channel region is formedin the oxide semiconductor film has little variation in electricalcharacteristics and high reliability in some cases.

Each of the oxide semiconductor films 409 a, 409 b, and 409 c may have anon-single-crystal structure. The non-single-crystal structure includesa c-axis aligned crystalline oxide semiconductor (CAAC-OS) describedlater, a polycrystalline structure, a microcrystalline structuredescribed later, or an amorphous structure, for example. Among thenon-single-crystal structures, the amorphous structure has the highestdensity of defect states, whereas the CAAC-OS has the lowest density ofdefect states.

Note that each of the oxide semiconductor films 409 a, 409 b, and 409 cmay be a single film or stacked films including two or more of thefollowing regions: a region having an amorphous structure, a regionhaving a microcrystalline structure, a region having a polycrystallinestructure, a CAAC-OS region, and a region having a single-crystalstructure.

[Liquid Crystal Layer]

As examples of the liquid crystal layer 620, thermotropic liquidcrystal, low-molecular liquid crystal, high-molecular liquid crystal,polymer dispersed liquid crystal, ferroelectric liquid crystal, andanti-ferroelectric liquid crystal can be given. Alternatively, a liquidcrystal material which exhibits a cholesteric phase, a smectic phase, acubic phase, a chiral nematic phase, an isotropic phase, or the like maybe used. Further, a liquid crystal material exhibiting a blue phase maybe used.

For a driving method of the liquid crystal layer 620, an in-planeswitching (IPS) mode, a twisted nematic (TN) mode, a fringe fieldswitching (FFS) mode, an axially symmetric aligned micro-cell (ASM)mode, an optically compensated birefringence (OCB) mode, a ferroelectricliquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC)mode, or the like can be used. In addition, the liquid crystal layer 620can be driven by, for example, a vertical alignment (VA) mode such as amulti-domain vertical alignment (MVA) mode, a patterned verticalalignment (PVA) mode, an electrically controlled birefringence (ECB)mode, a continuous pinwheel alignment (CPA) mode, or an advanced superview (ASV) mode can be used.

[EL Layer]

The EL layer 419 includes at least a light-emitting material. Examplesof the light-emitting material include an organic compound and aninorganic compound such as a quantum dot.

The organic compound and the inorganic compound can be formed by anevaporation method (including a vacuum evaporation method), an ink-jetmethod, a coating method, gravure printing, or the like, for example.

Examples of materials that can be used for the organic compound includea fluorescent material and a phosphorescent material. A fluorescentmaterial is preferably used in terms of the lifetime, while afluorescent material is preferably used in terms of the efficiency.Furthermore, both of a phosphorescent material and a phosphorescentmaterial may be used.

A quantum dot is a semiconductor nanocrystal with a size of severalnanometers and contains approximately 1×10³ to 1×10⁶ atoms. Since energyshift of quantum dots depend on their size, quantum dots made of thesame substance emit light with different wavelengths depending on theirsize; thus, emission wavelengths can be easily adjusted by changing thesize of quantum dots.

Since a quantum dot has an emission spectrum with a narrow peak,emission with high color purity can be obtained. In addition, a quantumdot is said to have a theoretical internal quantum efficiency ofapproximately 100%, which far exceeds that of a fluorescent organiccompound, i.e., 25%, and is comparable to that of a phosphorescentorganic compound. Therefore, a quantum dot can be used as alight-emitting material to obtain a light-emitting element having highlight-emitting efficiency. Furthermore, since a quantum dot which is aninorganic compound has high inherent stability, a light-emitting elementwhich is favorable also in terms of lifetime can be obtained.

Examples of a material of a quantum dot include a Group 14 element inthe periodic table, a Group 15 element in the periodic table, a Group 16element in the periodic table, a compound of a plurality of Group 14elements in the period table, a compound of an element belonging to anyof Groups 4 to 14 in the periodic table and a Group 16 element in theperiod table, a compound of a Group 2 element in the periodic table anda Group 16 element in the period table, a compound of a Group 13 elementin the period table and a Group 15 element in the period table, acompound of a Group 13 element in the period table and a Group 17element in the period table, a compound of a Group 14 element in theperiod table and a Group 15 element in the period table, a compound of aGroup 11 element in the period table and a Group 17 element in theperiod table, iron oxides, titanium oxides, spinel chalcogenides, andsemiconductor clusters.

Specific examples include, but are not limited to, cadmium selenide;cadmium sulfide; cadmium telluride; zinc selenide; zinc oxide; zincsulfide; zinc telluride; mercury sulfide; mercury selenide; mercurytelluride; indium arsenide; indium phosphide; gallium arsenide; galliumphosphide; indium nitride; gallium nitride; indium antimonide; galliumantimonide; aluminum phosphide; aluminum arsenide; aluminum antimonide;lead selenide; lead telluride; lead sulfide; indium selenide; indiumtelluride; indium sulfide; gallium selenide; arsenic sulfide; arsenicselenide; arsenic telluride; antimony sulfide; antimony selenide;antimony telluride; bismuth sulfide; bismuth selenide; bismuthtelluride; silicon; silicon carbide; germanium; tin; selenium;tellurium; boron; carbon; phosphorus; boron nitride; boron phosphide;boron arsenide; aluminum nitride; aluminum sulfide; barium sulfide;barium selenide; barium telluride; calcium sulfide; calcium selenide;calcium telluride; beryllium sulfide; beryllium selenide; berylliumtelluride; magnesium sulfide; magnesium selenide; germanium sulfide;germanium selenide; germanium telluride; tin sulfide; tin selenide; tintelluride; lead oxide; copper fluoride; copper chloride; copper bromide;copper iodide; copper oxide; copper selenide; nickel oxide; cobaltoxide; cobalt sulfide; triiron tetraoxide; iron sulfide; manganeseoxide; molybdenum sulfide; vanadium oxide; tungsten oxide; tantalumoxide; titanium oxide; zirconium oxide; silicon nitride; germaniumnitride; aluminum oxide; barium titanate; a compound of selenium, zinc,and cadmium; a compound of indium, arsenic, and phosphorus; a compoundof cadmium, selenium, and sulfur; a compound of cadmium, selenium, andtellurium; a compound of indium, gallium, and arsenic; a compound ofindium, gallium, and selenium; a compound of indium, selenium, andsulfur; a compound of copper, indium, and sulfur; and combinationsthereof. What is called an alloyed quantum dot, whose composition isrepresented by a given ratio, may be used. For example, an alloyedquantum dot of cadmium, selenium, and sulfur is a means effective inobtaining blue light because the emission wavelength can be changed bychanging the content ratio of elements.

As the quantum dot, any of a core-type quantum dot, a core-shell quantumdot, a core-multishell quantum dot, and the like can be used. Note thatwhen a core is covered with a shell formed of another inorganic materialhaving a wider band gap, the influence of defects and dangling bondsexisting at the surface of a nanocrystal can be reduced. Since such astructure can significantly improve the quantum efficiency of lightemission, it is preferable to use a core-shell or core-multishellquantum dot. Examples of the material of a shell include zinc sulfideand zinc oxide.

Quantum dots have a high proportion of surface atoms and thus have highreactivity and easily cohere together. For this reason, it is preferablethat a protective agent be attached to, or a protective group beprovided at the surfaces of quantum dots. The attachment of theprotective agent or the provision of the protective group can preventcohesion and increase solubility in a solvent. It can also reducereactivity and improve electrical stability. Examples of the protectiveagent (or the protective group) include polyoxyethylene alkyl etherssuch as polyoxyethylene lauryl ether, polyoxyethylene stearyl ether, andpolyoxyethylene oleyl ether; trialkylphosphines such astripropylphosphine, tributylphosphine, trihexylphosphine, andtrioctylphoshine; polyoxyethylene alkylphenyl ethers such aspolyoxyethylene n-octylphenyl ether and polyoxylethylene n-nonylphenylether; tertiary amines such as tri(n-hexyl)amine, tri(n-octyl)amine, andtri(n-decyl)amine; organophosphorus compounds such as tripropylphosphineoxide, tributylphosphine oxide, trihexylphosphine oxide,trioctylphosphine oxide, and tridecylphosphine oxide; polyethyleneglycol diesters such as polyethylene glycol dilaurate and polyethyleneglycol distearate; organic nitrogen compounds such asnitrogen-containing aromatic compounds, e.g., pyridines, lutidines,collidines, and quinolones; animoalkanes such as hexylamine, octylamine,decylamine, dodecylamine, tetradecylamine, hexadecylamine, andoctadecylamine; dialkylsulfides such as dibutylsulfide;dialkylsulfoxides such as dimethylsulfoxide and dibutylsulfoxide;organic sulfur compounds such as sulfur-containing aromatic compounds,e.g., thiophene; higher fatty acids such as a palmitin acid, a stearicacid, and an oleic acid; alcohols; sorbitan fatty acid esters; fattyacid modified polyesters; tertiary amine modified polyurethanes; andpolyethyleneimines.

Since band gaps of quantum dots are increased as their size isdecreased, the size is adjusted as appropriate so that light with adesired wavelength can be obtained. Light emission from the quantum dotsis shifted to a blue color side, i.e., a high energy side, as thecrystal size is decreased; thus, emission wavelengths of the quantumdots can be adjusted over wavelength regions of spectra of anultraviolet region, a visible light region, and an infrared region bychanging the size of quantum dots. The range of size (diameter) ofquantum dots which is usually used is 0.5 nm to 20 nm, preferably 1 nmto 10 nm. The emission spectra are narrowed as the size distribution ofthe quantum dots gets smaller, and thus light can be obtained with highcolor purity. The shape of the quantum dots is not particularly limitedand may be spherical shape, a rod shape, a circular shape, or the like.Quantum rods which are rod-like shape quantum dots emit directionallight polarized in the c-axis direction; thus, quantum rods can be usedas a light-emitting material to obtain a light-emitting element withhigher external quantum efficiency.

In most EL elements, to improve luminous efficiency, light-emittingmaterials are dispersed in host materials and the host materials need tobe substances each having a singlet excitation energy or a tripletexcitation energy higher than or equal to that of the light-emittingmaterial. In the case of using a blue phosphorescent material, it isparticularly difficult to develop a host material which has a tripletexcitation energy higher than or equal to that of the bluephosphorescent material and which is excellent in terms of a lifetime.On the other hand, even when a light-emitting layer is composed ofquantum dots and made without a host material, the quantum dots enableluminous efficiency to be ensured; thus, a light-emitting element whichis favorable in terms of a lifetime can be obtained. In the case wherethe light-emitting layer is composed of quantum dots, the quantum dotspreferably have core-shell structures (including core-multishellstructures).

[Alignment Film]

For the alignment films 618 a and 618 b, a material containing polyimideor the like can be used. Specifically, a material containing polyimideor the like may be subjected to a rubbing process or an opticalalignment process to have alignment in a predetermined direction.

[Light-Blocking Film]

The light-blocking film 602 functions as a so-called black matrix. Forthe light-blocking film 602, a material that prevents light transmissionis used. Examples of the material that prevents light transmissioninclude a metal material and an organic resin material containing ablack pigment.

[Coloring Film]

The coloring film 604 functions as a so-called color filter. For thecoloring film 604, a material transmitting light of a predeterminedcolor (e.g., a material transmitting light of blue, green, red, yellow,or white) is used.

[Structure Body]

The structure bodies 610 a and 610 b have a function of providing acertain space between components between which the structure bodies 610a and 610 b are interposed. For each of the structure bodies 610 a and610 b, an organic material, an inorganic material, or a compositematerial of an organic material and an inorganic material can be used.For the inorganic material and the organic material, the materials forthe insulating films 404, 406, 408, 410 a, 410 b, 410 c, 412, 413, 416,418, and 606 can be used.

[Functional Film]

As the functional film 626, a polarizing plate, a retardation plate, adiffusing film, an anti-reflective film, a condensing film, or the likecan be used. As the functional film 626, an antistatic film preventingthe attachment of a foreign substance, a water repellent filmsuppressing the attachment of stain, a hard coat film suppressinggeneration of a scratch in use, or the like can be used.

[Sealant]

For the sealant 454, an inorganic material, an organic material, acomposite material of an inorganic material and an organic material, orthe like can be used. Examples of the organic material include athermally fusible resin and a curable resin. As the sealant 454, anadhesive including a resin material (e.g., a reactive curable adhesive,a photocurable adhesive, a thermosetting adhesive, or an anaerobicadhesive) may be used. Examples of such resin materials include an epoxyresin, an acrylic resin, a silicone resin, a phenol resin, a polyimideresin, an imide resin, a polyvinyl chloride (PVC) resin, a polyvinylbutyral (PVB) resin, and an ethylene vinyl acetate (EVA) resin.

[Sealant]

For the sealant 622, the materials for the sealant 454 can be used. Forthe sealant 622, a material such as glass frit may be used in additionto the above materials. For a material used for the sealant 622, amaterial which is impermeable to moisture or oxygen is preferably used.

As described above, the display device of one embodiment of the presentinvention includes two display elements. Furthermore, the display deviceincludes two transistors for driving the two display elements. Onedisplay element functions as a reflective liquid crystal element and theother display element functions as a transmissive EL element; thus, anovel display device that is highly convenient or reliable can beprovided. With use of oxide semiconductor films for channel regions ofthe transistors for driving the display elements and one electrode ofeach of the two display elements, a novel display device in whichmanufacturing cost is reduced can be provided. In addition, when each ofthe transistors has a staggered structure, parasitic capacitancegenerated between the gate electrode and the source and drain electrodescan be reduced, whereby a novel display device with low powerconsumption can be provided.

Note that the structure described in this embodiment can be used inappropriate combination with the structure described in any of the otherembodiments.

Embodiment 2

In this embodiment, a transistor that can be used for the display deviceof one embodiment of the present invention and a method formanufacturing the transistor will be described with reference to FIGS.19A to 19C, FIGS. 20A to 20C, FIGS. 21A and 21B, FIGS. 22A and 22B,FIGS. 23A and 23B, FIGS. 24A and 24B, FIGS. 25A and 25B, FIGS. 26A and26B, FIGS. 27A and 27B, FIGS. 28A and 28B, FIGS. 29A and 29B, FIGS. 30Ato 30C, FIGS. 31A to 31D, FIGS. 32A to 32C, FIGS. 33A and 33B, FIGS. 34Ato 34D, FIGS. 35A to 35C, and FIGS. 36A to 36C.

<2-1. Structure Example 1 of Transistor>

FIGS. 19A to 19C show an example of a transistor. Note that thetransistor in FIGS. 19A to 19C has a staggered (top-gate) structure.

FIG. 19A is a top view of a transistor 100. FIG. 19B is across-sectional view taken along a dashed-dotted line X1-X2 in FIG. 19A.FIG. 19C is a cross-sectional view taken along a dashed-dotted lineY1-Y2 in FIG. 19A. For clarity, FIG. 19A does not illustrate somecomponents such as an insulating film 110. As in FIG. 19A, somecomponents are not illustrated in some cases in top views of transistorsdescribed below. Furthermore, the direction of the dashed-dotted lineX1-X2 may be referred to as a channel length (L) direction, and thedirection of the dashed-dotted line Y1-Y2 may be referred to as achannel width (W) direction.

The transistor 100 illustrated in FIGS. 19A to 19C includes aninsulating film 104 over a substrate 102; an oxide semiconductor film108 over the insulating film 104; the insulating film 110 over the oxidesemiconductor film 108; a conductive film 112 over the insulating film110; and an insulating film 116 over the insulating film 104, the oxidesemiconductor film 108, and the conductive film 112. Note that the oxidesemiconductor film 108 includes a channel region 108 i overlapping withthe conductive film 112, a source region 108 s in contact with theinsulating film 116, and a drain region 108 d in contact with theinsulating film 116.

Furthermore, the insulating film 116 contains nitrogen or hydrogen. Theinsulating film 116 is in contact with the source region 108 s and thedrain region 108 d, so that nitrogen or hydrogen that is contained inthe insulating film 116 is added to the source region 108 s and thedrain region 108 d. The source region 108 s and the drain region 108 deach have a high carrier density when nitrogen or hydrogen is addedthereto.

The transistor 100 may further include an insulating film 118 over theinsulating film 116, a conductive film 120 a electrically connected tothe source region 108 s through an opening 141 a provided in theinsulating films 116 and 118, and a conductive film 120 b electricallyconnected to the drain region 108 d through an opening 141 b provided inthe insulating films 116 and 118.

In this specification and the like, the insulating film 104 may bereferred to as a first insulating film, the insulating film 110 may bereferred to as a second insulating film, the insulating film 116 may bereferred to as a third insulating film, and the insulating film 118 maybe referred to as a fourth insulating film. The conductive film 112functions as a gate electrode, the conductive film 120 a functions as asource electrode, and the conductive film 120 b functions as a drainelectrode.

The insulating film 110 functions as a gate insulating film. Theinsulating film 110 includes an excess oxygen region. Since theinsulating film 110 includes the excess oxygen region, excess oxygen canbe supplied to the channel region 108 i included in the oxidesemiconductor film 108. As a result, oxygen vacancies that might beformed in the channel region 108 i can be filled with excess oxygen,which can provide a highly reliable semiconductor device.

To supply excess oxygen to the oxide semiconductor film 108, excessoxygen may be supplied to the insulating film 104 that is formed underthe oxide semiconductor film 108. However, in that case, excess oxygencontained in the insulating film 104 might also be supplied to thesource region 108 s and the drain region 108 d included in the oxidesemiconductor film 108. When excess oxygen is supplied to the sourceregion 108 s and the drain region 108 d, the resistance of the sourceregion 108 s and the drain region 108 d might be increased.

In contrast, in the structure in which the insulating film 110 formedover the oxide semiconductor film 108 contains excess oxygen, excessoxygen can be selectively supplied only to the channel region 108 i.Alternatively, the carrier density of the source and drain regions 108 sand 108 d can be selectively increased after excess oxygen is suppliedto the channel region 108 i and the source and drain regions 108 s and108 d, in which case an increase in the resistance of the source anddrain regions 108 s and 108 d can be prevented.

Furthermore, each of the source region 108 s and the drain region 108 dincluded in the oxide semiconductor film 108 preferably contains anelement that forms an oxygen vacancy or an element that is bonded to anoxygen vacancy. Typical examples of the element that forms an oxygenvacancy or the element that is bonded to an oxygen vacancy includehydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur,chlorine, titanium, and a rare gas. Typical examples of the rare gaselement are helium, neon, argon, krypton, and xenon. The element thatforms an oxygen vacancy is diffused from the insulating film 116 to thesource region 108 s and the drain region 108 d in the case where theinsulating film 116 contains one or more such elements. In addition oralternatively, the element that forms an oxygen vacancy is added to thesource region 108 s and the drain region 108 d by impurity additiontreatment.

An impurity element added to the oxide semiconductor film cuts a bondbetween a metal element and oxygen in the oxide semiconductor film, sothat an oxygen vacancy is formed. Alternatively, when an impurityelement is added to the oxide semiconductor film, oxygen bonded to ametal element in the oxide semiconductor film is bonded to the impurityelement and detached from the metal element, so that an oxygen vacancyis formed. As a result, the oxide semiconductor film has a highercarrier density, and thus, the conductivity thereof becomes higher.

Next, details of the components of the semiconductor device in FIGS. 19Ato 19C will be described.

[Substrate]

As the substrate 102, any of a variety of substrates can be used withoutparticular limitation. The substrate 102 can be formed using a materialsimilar to that of the substrates 401, 452, and 652 described inEmbodiment 1.

[First Insulating Film]

The insulating film 104 can be formed by a sputtering method, a CVDmethod, an evaporation method, a pulsed laser deposition (PLD) method, aprinting method, a coating method, or the like as appropriate. Forexample, the insulating film 104 can be formed to have a single-layerstructure or stacked-layer structure of an oxide insulating film and/ora nitride insulating film. To improve the properties of the interfacewith the oxide semiconductor film 108, at least a region of theinsulating film 104 which is in contact with the oxide semiconductorfilm 108 is preferably formed using an oxide insulating film. When theinsulating film 104 is formed using an oxide insulating film from whichoxygen is released by heating, oxygen contained in the insulating film104 can be moved to the oxide semiconductor film 108 by heat treatment.

The thickness of the insulating film 104 can be greater than or equal to50 nm, greater than or equal to 100 nm and less than or equal to 3000nm, or greater than or equal to 200 nm and less than or equal to 1000nm. By increasing the thickness of the insulating film 104, the amountof oxygen released from the insulating film 104 can be increased, andinterface states at the interface between the insulating film 104 andthe oxide semiconductor film 108 and oxygen vacancies included in thechannel region 108 i of the oxide semiconductor film 108 can be reduced.

For example, the insulating film 104 can be formed to have asingle-layer structure or stacked-layer structure of silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, aluminumoxide, hafnium oxide, gallium oxide, a Ga—Zn oxide, or the like. In thisembodiment, the insulating film 104 has a stacked-layer structure of asilicon nitride film and a silicon oxynitride film. With the insulatingfilm 104 having such a stack-layer structure including a silicon nitridefilm as a lower layer and a silicon oxynitride film as an upper layer,oxygen can be efficiently introduced into the oxide semiconductor film108.

[Oxide Semiconductor Film]

The oxide semiconductor film 108 can be formed using a material similarto that of the oxide semiconductor films 409 a, 409 b, and 409 cdescribed in Embodiment 1.

[Second Insulating Film]

The insulating film 110 functions as a gate insulating film of thetransistor 100. In addition, the insulating film 110 has a function ofsupplying oxygen to the oxide semiconductor film 108, particularly tothe channel region 108 i. The insulating film 110 can be formed to havea single-layer structure or a stacked-layer structure of an oxideinsulating film or a nitride insulating film, for example. To improvethe interface properties with the oxide semiconductor film 108, a regionwhich is in the insulating film 110 and in contact with the oxidesemiconductor film 108 is preferably formed using at least an oxideinsulating film. For example, silicon oxide, silicon oxynitride, siliconnitride oxide, or silicon nitride may be used for the insulating film110.

The thickness of the insulating film 110 can be greater than or equal to5 nm and less than or equal to 400 nm, greater than or equal to 5 nm andless than or equal to 300 nm, or greater than or equal to 10 nm and lessthan or equal to 250 nm.

It is preferable that the insulating film 110 have few defects andtypically have as few signals observed by electron spin resonance (ESR)spectroscopy as possible. Examples of the signals include a signal dueto an E′ center observed at a g-factor of 2.001. Note that the E′ centeris due to the dangling bond of silicon. As the insulating film 110, asilicon oxide film or a silicon oxynitride film whose spin density of asignal due to the E′ center is lower than or equal to 3×10¹⁷ spins/cm³and preferably lower than or equal to 5×10¹⁶ spins/cm³ may be used.

In addition to the above-described signal, a signal due to nitrogendioxide (NO₂) might be observed in the insulating film 110. The signalis divided into three signals according to the N nuclear spin; a firstsignal, a second signal, and a third signal. The first signal isobserved at a g-factor of greater than or equal to 2.037 and less thanor equal to 2.039. The second signal is observed at a g-factor ofgreater than or equal to 2.001 and less than or equal to 2.003. Thethird signal is observed at a g-factor of greater than or equal to 1.964and less than or equal to 1.966.

It is suitable to use an insulating film whose spin density of a signaldue to nitrogen dioxide (NO₂) is higher than or equal to 1×10¹⁷spins/cm³ and lower than 1×10¹⁸ spins/cm³ as the insulating film 110,for example.

Note that a nitrogen oxide (NO_(X)) such as a nitrogen dioxide (NO₂)forms a level in the insulating film 110. The level is positioned in theenergy gap of the oxide semiconductor film 108. Thus, when nitrogenoxide (NO_(x)) is diffused to the interface between the insulating film110 and the oxide semiconductor film 108, an electron might be trappedby the level on the insulating film 110 side. As a result, the trappedelectron remains in the vicinity of the interface between the insulatingfilm 110 and the oxide semiconductor film 108, leading to a positiveshift of the threshold voltage of the transistor. Accordingly, the useof a film with a low nitrogen oxide content as the insulating film 110can reduce a shift of the threshold voltage of the transistor.

As an insulating film that releases a small amount of nitrogen oxide(NO_(x)), for example, a silicon oxynitride film can be used. Thesilicon oxynitride film releases more ammonia than nitrogen oxide(NO_(x)) in thermal desorption spectroscopy (TDS); the typical releasedamount of ammonia is greater than or equal to 1×10¹⁸ molecules cm⁻⁻³ andless than or equal to 5×10¹⁹ molecules cm⁻⁻³. Note that the releasedamount of ammonia is the total amount of ammonia released by heattreatment in a range of 50° C. to 650° C. or 50° C. to 550° C. in TDS.

Since nitrogen oxide (NO_(x)) reacts with ammonia and oxygen in heattreatment, the use of an insulating film that releases a large amount ofammonia reduces nitrogen oxide (NO_(x)).

Note that in the case where the insulating film 110 is analyzed by SIMS,nitrogen concentration in the film is preferably lower than or equal to6×10²⁰ atoms/cm³.

The insulating film 110 may be formed using a high-k material such ashafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen isadded (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added(HfAl_(x)O_(y)N_(z)), or hafnium oxide. The use of such a high-kmaterial enables a reduction in gate leakage current of a transistor.

[Third Insulating Film]

The insulating film 116 contains nitrogen or hydrogen. The insulatingfilm 116 may contain fluorine. As the insulating film 116, for example,a nitride insulating film can be used. The nitride insulating film canbe formed using silicon nitride, silicon nitride oxide, siliconoxynitride, silicon nitride fluoride, silicon fluoronitride, or thelike. The hydrogen concentration in the insulating film 116 ispreferably higher than or equal to 1×10²² atoms/cm³. Furthermore, theinsulating film 116 is in contact with the source region 108 s and thedrain region 108 d of the oxide semiconductor film 108. Thus, theconcentration of an impurity (nitrogen or hydrogen) in the source region108 s and the drain region 108 d in contact with the insulating film 116is increased, leading to an increase in the carrier density of thesource region 108 s and the drain region 108 d.

[Fourth Insulating Film]

As the insulating film 118, an oxide insulating film can be used.Alternatively, a stack including an oxide insulating film and a nitrideinsulating film can be used as the insulating film 118. The insulatingfilm 118 can be formed using, for example, silicon oxide, siliconoxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide,gallium oxide, or Ga—Zn oxide.

Furthermore, the insulating film 118 preferably functions as a barrierfilm against hydrogen, water, and the like from the outside.

The thickness of the insulating film 118 can be greater than or equal to30 nm and less than or equal to 500 nm, or greater than or equal to 100nm and less than or equal to 400 nm.

[Conductive Film]

The conductive films 112, 120 a, and 120 b can be formed by a sputteringmethod, a vacuum evaporation method, a pulsed laser deposition (PLD)method, a thermal CVD method, or the like. The conductive films 112, 120a, and 120 b can be formed using materials similar to those of theconductive films 402, 403 a, 403 b, 403 c, 405 a, 405 b, 405 c, 405 d,407 a, 407 b, 407 c, 407 d, 407 e, 414 a, 414 b, 414 c, 414 d, 414 e 414f, 414 g, 414 h, 417, 420, and 608 and the oxide semiconductor films 411a, 411 b, and 411 c which are described in Embodiment 1.

The conductive films 112, 120 a, and 120 b can also be formed using alight-transmitting conductive material such as ITO, indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium zinc oxide, or ITSO. It is also possible to havea layered structure formed using the above light-transmitting conductivematerial and the above metal element.

Note that an oxide semiconductor typified by an In—Ga—Zn oxide may beused for the conductive film 112. The oxide semiconductor can have ahigh carrier density when nitrogen or hydrogen is supplied from theinsulating film 116. In other words, the oxide semiconductor functionsas an oxide conductor (OC). Accordingly, the oxide semiconductor can beused for a gate electrode.

The conductive film 112 can have, for example, a single-layer structureof an oxide conductor (OC), a single-layer structure of a metal film, ora stacked-layer structure of an oxide conductor (OC) and a metal film.

Note that it is suitable that the conductive film 112 has a single-layerstructure of a light-shielding metal film or a stacked-layer structureof an oxide conductor (OC) and a light-shielding metal film because thechannel region 108 i formed under the conductive film 112 can beshielded from light. In the case where the conductive film 112 has astacked-layer structure of an oxide semiconductor or an oxide conductor(OC) and a light-shielding metal film, formation of a metal film (e.g.,a titanium film or a tungsten film) over the oxide semiconductor or theoxide conductor (OC) produces any of the following effects: theresistance of the oxide semiconductor or the oxide conductor (OC) isreduced by the diffusion of the constituent element of the metal film tothe oxide semiconductor or oxide conductor (OC) side, the resistance isreduced by damage (e.g., sputtering damage) during the deposition of themetal film, and the resistance is reduced when oxygen vacancies areformed by the diffusion of oxygen in the oxide semiconductor or theoxide conductor (OC) to the metal film.

The thickness of the conductive films 112, 120 a, and 120 b can begreater than or equal to 30 nm and less than or equal to 500 nm, orgreater than or equal to 100 nm and less than or equal to 400 nm.

<2-2. Structure Example 2 of Semiconductor Device>

Next, a structure of a transistor different from that in FIGS. 19A to19C will be described with reference to FIGS. 20A to 20C.

FIG. 20A is a top view of a transistor 100A. FIG. 20B is across-sectional view taken along the dashed-dotted line X1-X2 in FIG.20A. FIG. 20C is a cross-sectional view taken along the dashed-dottedline Y1-Y2 in FIG. 20A.

The transistor 100A illustrated in FIGS. 20A to 20C includes aconductive film 106 over the substrate 102; the insulating film 104 overthe conductive film 106; the oxide semiconductor film 108 over theinsulating film 104; the insulating film 110 over the oxidesemiconductor film 108; the conductive film 112 over the insulating film110; and the insulating film 116 over the insulating film 104, the oxidesemiconductor film 108, and the conductive film 112. Note that the oxidesemiconductor film 108 includes the channel region 108 i overlappingwith the conductive film 112, the source region 108 s in contact withthe insulating film 116, and the drain region 108 d in contact with theinsulating film 116.

The transistor 100A includes the conductive film 106 and an opening 143in addition to the components of the transistor 100 described above.

Note that the opening 143 is provided in the insulating films 104 and110. The conductive film 106 is electrically connected to the conductivefilm 112 through the opening 143. Thus, the same potential is applied tothe conductive film 106 and the conductive film 112. Note that differentpotentials may be applied to the conductive film 106 and the conductivefilm 112 without providing the opening 143. Alternatively, theconductive film 106 may be used as a light-shielding film withoutproviding the opening 143. When the conductive film 106 is formed usinga light-shielding material, for example, light irradiating the channelregion 108 i from the bottom can be reduced.

In the case of the structure of the transistor 100A, the conductive film106 functions as a first gate electrode (also referred to as abottom-gate electrode), the conductive film 112 functions as a secondgate electrode (also referred to as a top-gate electrode), theinsulating film 104 functions as a first gate insulating film, and theinsulating film 110 functions as a second gate insulating film.

The conductive film 106 can be formed using a material similar to theabove-described materials of the conductive films 112, 120 a, and 120 b.It is particularly suitable to use a material containing copper for theconductive film 106 because the resistance can be reduced. It issuitable that, for example, each of the conductive films 106, 120 a, and120 b has a stacked-layer structure in which a copper film is over atitanium nitride film, a tantalum nitride film, or a tungsten film. Inthat case, when the transistor 100A is used as a pixel transistor and/ora driving transistor of a display device, parasitic capacitancegenerated between the conductive films 106 and 120 a and between theconductive films 106 and 120 b can be reduced. Thus, the conductivefilms 106, 120 a, and 120 b can be used not only as the first gateelectrode, the source electrode, and the drain electrode of thetransistor 100A, but also as power source supply wirings, signal supplywirings, connection wirings, or the like of the display device.

In this manner, unlike the transistor 100 described above, thetransistor 100A in FIGS. 20A to 20C has a structure in which aconductive film functioning as a gate electrode is provided over andunder the oxide semiconductor film 108. As in the transistor 100A, asemiconductor device of one embodiment of the present invention may havea plurality of gate electrodes.

As illustrated in FIG. 20C, the oxide semiconductor film 108 faces theconductive film 106 functioning as a first gate electrode and theconductive film 112 functioning as a second gate electrode and ispositioned between the two conductive films functioning as the gateelectrodes.

Furthermore, the length of the conductive film 112 in the channel widthdirection is larger than the length of the oxide semiconductor film 108in the channel width direction. In the channel width direction, thewhole oxide semiconductor film 108 is covered with the conductive film112 with the insulating film 110 placed therebetween. Since theconductive film 112 is connected to the conductive film 106 through theopening 143 provided in the insulating films 104 and 110, a side surfaceof the oxide semiconductor film 108 in the channel width direction facesthe conductive film 112 with the insulating film 110 placedtherebetween.

In other words, in the channel width direction of the transistor 100A,the conductive films 106 and 112 are connected to each other through theopening 143 provided in the insulating films 104 and 110, and theconductive films 106 and 112 surround the oxide semiconductor film 108with the insulating films 104 and 110 placed therebetween.

Such a structure enables the oxide semiconductor film 108 included inthe transistor 100A to be electrically surrounded by electric fields ofthe conductive film 106 functioning as a first gate electrode and theconductive film 112 functioning as a second gate electrode. A devicestructure of a transistor, like that of the transistor 100A, in whichelectric fields of a first gate electrode and a second gate electrodeelectrically surround an oxide semiconductor film in which a channelregion is formed can be referred to as a surrounded channel (S-channel)structure.

Since the transistor 100A has the S-channel structure, an electric fieldfor inducing a channel can be effectively applied to the oxidesemiconductor film 108 by the conductive film 106 or the conductive film112; thus, the current drive capability of the transistor 100A can beimproved and high on-state current characteristics can be obtained. As aresult of the high on-state current, it is possible to reduce the sizeof the transistor 100A. Furthermore, since the transistor 100A has astructure in which the oxide semiconductor film 108 is surrounded by theconductive film 106 and the conductive film 112, the mechanical strengthof the transistor 100A can be increased.

When seen in the channel width direction of the transistor 100A, anopening different from the opening 143 may be formed on the side of theoxide semiconductor film 108 on which the opening 143 is not formed.

When a transistor has a pair of gate electrodes between which asemiconductor film is positioned as in the transistor 100A, one of thegate electrodes may be supplied with a signal A, and the other gateelectrode may be supplied with a fixed potential V_(b). Alternatively,one of the gate electrodes may be supplied with the signal A, and theother gate electrode may be supplied with a signal B. Alternatively, oneof the gate electrodes may be supplied with a fixed potential V_(a), andthe other gate electrode may be supplied with the fixed potential V_(b).

The signal A is, for example, a signal for controlling the on/off state.The signal A may be a digital signal with two kinds of potentials, apotential V1 and a potential V2 (V1>V2). For example, the potential V1can be a high power supply potential, and the potential V2 can be a lowpower supply potential. The signal A may be an analog signal.

The fixed potential V_(b) is, for example, a potential for controlling athreshold voltage V_(thA) of the transistor. The fixed potential V_(b)may be the potential V1 or the potential V2. In that case, a potentialgenerator circuit for generating the fixed potential V_(b) is notnecessary, which is preferable. The fixed potential V_(b) may bedifferent from the potential V1 or the potential V2. When the fixedpotential V_(b) is low, the threshold voltage V_(thA) can be high insome cases. As a result, the drain current flowing when the gate-sourcevoltage V_(gs) is 0 V can be reduced, and leakage current in a circuitincluding the transistor can be reduced in some cases. The fixedpotential V_(b) may be, for example, lower than the low power supplypotential. Meanwhile, a high fixed potential V_(b) can lower thethreshold voltage V_(thA) in some cases. As a result, the drain currentflowing when the gate-source voltage V_(gs) is a high power supplypotential and the operating speed of the circuit including thetransistor can be increased in some cases. The fixed potential V_(b) maybe, for example, higher than the low power supply potential.

The signal B is, for example, a signal for controlling the on/off state.The signal B may be a digital signal with two kinds of potentials, apotential V3 and a potential V4 (V3>V4). For example, the potential V3can be a high power supply potential, and the potential V4 can be a lowpower supply potential. The signal B may be an analog signal.

When both the signal A and the signal B are digital signals, the signalB may have the same digital value as the signal A. In this case, it maybe possible to increase the on-state current of the transistor and theoperating speed of the circuit including the transistor. Here, thepotential V1 and the potential V2 of the signal A may be different fromthe potential V3 and the potential V4 of the signal B. For example, if agate insulating film for the gate to which the signal B is input isthicker than a gate insulating film for the gate to which the signal Ais input, the potential amplitude of the signal B (V3-V4) may be largerthan the potential amplitude of the signal A (V1-V2). In this manner,the influence of the signal A and that of the signal B on the on/offstate of the transistor can be substantially the same in some cases.

When both the signal A and the signal B are digital signals, the signalB may have a digital value different from that of the signal A. In thiscase, the signal A and the signal B can separately control thetransistor, and thus, higher performance can be achieved. The transistorwhich is, for example, an n-channel transistor can function by itself asa NAND circuit, a NOR circuit, or the like in the following case: thetransistor is turned on only when the signal A has the potential V1 andthe signal B has the potential V3, or the transistor is turned off onlywhen the signal A has the potential V2 and the signal B has thepotential V4. The signal B may be a signal for controlling the thresholdvoltage V_(thA). For example, the potential of the signal B in a periodin which the circuit including the transistor operates may be differentfrom the potential of the signal B in a period in which the circuit doesnot operate. The potential of the signal B may vary depending on theoperation mode of the circuit. In this case, the potential of the signalB is not changed as frequently as the potential of the signal A in somecases.

When both the signal A and the signal B are analog signals, the signal Bmay be an analog signal having the same potential as the signal A, ananalog signal whose potential is a constant times the potential of thesignal A, an analog signal whose potential is higher or lower than thepotential of the signal A by a constant, or the like. In this case, itmay be possible to increase the on-state current of the transistor andthe operating speed of the circuit including the transistor. The signalB may be an analog signal different from the signal A. In this case, thesignal A and the signal B can separately control the transistor, andthus, higher performance can be achieved.

The signal A may be a digital signal, and the signal B may be an analogsignal. Alternatively, the signal A may be an analog signal, and thesignal B may be a digital signal.

When both of the gate electrodes of the transistor are supplied with thefixed potentials, the transistor can function as an element equivalentto a resistor in some cases. For example, in the case where thetransistor is an n-channel transistor, the effective resistance of thetransistor can be sometimes low (high) when the fixed potential V_(a) orthe fixed potential V_(b) is high (low). When both the fixed potentialV_(a) and the fixed potential V_(b) are high (low), the effectiveresistance can be lower (higher) than that of a transistor with only onegate in some cases.

Except for the above-mentioned points, the transistor 100A has astructure and an effect similar to those of the transistor 100 describedabove.

<2-3. Structure Example 3 of Semiconductor Device>

Next, structures of a transistor different from that in FIGS. 20A to 20Cwill be described with reference to FIGS. 21A and 21B, FIGS. 22A and22B, FIGS. 23A and 23B, and FIGS. 24A and 24B.

FIGS. 21A and 21B are cross-sectional views of a transistor 100B, FIGS.22A and 22B are cross-sectional views of a transistor 100C, FIGS. 23Aand 23B are cross-sectional views of a transistor 100D, and FIGS. 24Aand 24B are cross-sectional views of a transistor 100E. Note that topviews of the transistor 100B, the transistor 100C, the transistor 100D,and the transistor 100E are similar to that of the transistor 100Aillustrated in FIG. 20A and thus are not described here.

The transistor 100B illustrated in FIGS. 21A and 21B is different fromthe above-described transistor 100A in the shape of the insulating film110 and the conductive film 112. Specifically, in the cross section ofthe transistor in the channel length (L) direction, the shape of theinsulating film 110 and the conductive film 112 is a rectangle in thetransistor 100A but is a tapered shape in the transistor 100B. Morespecifically, in the cross section of the transistor in the channellength (L) direction, an upper end portion of the conductive film 112 inthe transistor 100A is substantially aligned with a lower end portion ofthe insulating film 110, whereas an upper end portion of the conductivefilm 112 in the transistor 100B is located inward from a lower endportion of the insulating film 110. In other words, a side end portionof the insulating film 110 is located outward from a side end portion ofthe conductive film 112.

To fabricate the transistor 100A, the conductive film 112 and theinsulating film 110 are collectively formed by a dry etching methodusing the same mask. To fabricate the transistor 100B, the conductivefilm 112 and the insulating film 110 are formed by a combination of awet etching method and a dry etching method using the same mask.

A structure like that of the transistor 100A is preferable because endportions of the source region 108 s and the drain region 108 d can besubstantially aligned with end portions of the conductive film 112.Meanwhile, a structure like that of the transistor 100B is preferablebecause the coverage with the insulating film 116 can be improved.

The transistor 100C illustrated in FIGS. 22A and 22B is different fromthe above-described transistor 100A in the shape of the conductive film112 and the insulating film 110. Specifically, in the cross section ofthe transistor 100C in the channel length (L) direction, a lower endportion of the conductive film 112 is not aligned with an upper endportion of the insulating film 110. The lower end portion of theconductive film 112 is located inward from the upper end portion of theinsulating film 110.

For example, the structure of the transistor 100C can be obtained in thefollowing manner: the conductive film 112 and the insulating film 110are formed by a wet etching method and a dry etching method,respectively, using the same mask.

With the structure of the transistor 100C, regions 108 f are formed inthe oxide semiconductor film 108 in some cases. The regions 108 f areformed between the channel region 108 i and the source region 108 s andbetween the channel region 108 i and the drain region 108 d.

The regions 108 f function as high-resistance regions or low-resistanceregions. The high-resistance regions have the same level of resistanceas the channel region 108 i and do not overlap with the conductive film112 functioning as a gate electrode. In the case where the regions 108 fare high-resistance regions, the regions 108 f function as offsetregions. To suppress a decrease in the on-state current of thetransistor 100C, the regions 108 f functioning as offset regions mayeach have a length of 1 μm or less in a cross section in the channellength (L) direction.

The low-resistance regions have a resistance that is lower than that ofthe channel region 108 i and higher than that of the source region 108 sand the drain region 108 d. In the case where the regions 108 f arelow-resistance regions, the regions 108 f function as lightly dopeddrain (LDD) regions. The regions 108 f functioning as LDD regions canrelieve an electric field in the drain region, thereby reducing a changein the threshold voltage of the transistor due to the electric field inthe drain region.

Note that in the case where the regions 108 f serve as LDD regions, forexample, the regions 108 f are formed by supplying nitrogen or hydrogenfrom the insulating film 116 to the regions 108 f or by adding animpurity element from above the conductive film 112 and the insulatingfilm 110 using the conductive film 112 and the insulating film 110 as amask so that the impurity element is added to the oxide semiconductorfilm 108 through the insulating film 110.

The transistor 100D illustrated in FIGS. 23A and 23B is different fromthe above-described transistor 100A in the shape of the conductive film112 and the insulating film 110. Specifically, in the cross section ofthe transistor 100D in the channel length (L) direction, a lower endportion of the conductive film 112 is not aligned with an upper endportion of the insulating film 110. More specifically, the lower endportion of the conductive film 112 is located outward from the upper endportion of the insulating film 110.

For example, the structure of the transistor 100D can be obtained in thefollowing manner: the conductive film 112 and the insulating film 110are formed by a dry etching method and a wet etching method,respectively, using the same mask.

With the structure of the transistor 100D, parts of the source region108 s and the drain region 108 d are provided inward from side surfacesof the conductive film 112 functioning as a gate electrode. Note that aregion where the conductive film 112 and the source region 108 s overlapwith each other and a region where the conductive film 112 and the drainregion 108 d overlap with each other function as what are called overlapregions (also referred to as Lov regions). Note that the Lov regionsoverlap with the conductive film 112 functioning as the gate electrodeand have lower resistance than the channel region 108 i. With the Lovregions, no high-resistance region is formed between the channel region108 i and the source region 108 s or the drain region 108 d;accordingly, the on-state current of the transistor can be increased.

The transistor 100E illustrated in FIGS. 24A and 24B is different fromthe above-described transistor 100A in that an insulating film 122functioning as a planarization film is provided over the insulating film118. The other components of the transistor 100E are similar to those ofthe transistor 100A described above and have similar effects.

The insulating film 122 has a function of covering unevenness and thelike caused by the transistor or the like. The insulating film 122 hasan insulating property and is formed using an inorganic material or anorganic material. Examples of the inorganic material include a siliconoxide film, a silicon oxynitride film, a silicon nitride oxide film, asilicon nitride film, an aluminum oxide film, and an aluminum nitridefilm. Examples of the organic material include photosensitive resinmaterials such as an acrylic resin and a polyimide resin.

Note that the size of each opening in the insulating film 122 is notlimited to that in FIGS. 24A and 24B, in which the openings are largerthan the openings 141 a and 141 b, and may be smaller than or equal tothe size of each of the openings 141 a and 141 b, for example.

In addition, the structure is not limited to the example in FIGS. 24Aand 24B, in which the conductive films 120 a and 120 b are provided overthe insulating film 122; for example, the insulating film 122 may beprovided over the conductive films 120 a and 120 b formed over theinsulating film 118.

<2-4. Structure Example 4 of Transistor>

Next, structures of a transistor different from that in FIGS. 20A to 20Cwill be described with reference to FIGS. 25A and 25B, FIGS. 26A and26B, FIGS. 27A and 27B, FIGS. 28A and 28B, FIGS. 29A and 29B, and FIGS.30A to 30C.

FIGS. 25A and 25B are cross-sectional views of a transistor 100F, FIGS.26A and 26B are cross-sectional views of a transistor 100G, FIGS. 27Aand 27B are cross-sectional views of a transistor 100H, FIGS. 28A and28B are cross-sectional views of a transistor 100J, and FIGS. 29A and29B are cross-sectional views of a transistor 100K. Note that top viewsof the transistor 100F, the transistor 100G, the transistor 100H, thetransistor 100J, and the transistor 100K are similar to that of thetransistor 100A illustrated in FIG. 20A and thus are not described here.

The transistors 100F, 100G, 100H, 100J, and 100K are different from theabove-described transistor 100A in the structure of the oxidesemiconductor film 108. The other components of the transistors 100F,100G, 100H, 100J, and 100K are similar to those of the transistor 100Adescribed above and have similar effects.

The oxide semiconductor film 108 of the transistor 100F illustrated inFIGS. 25A and 25B includes an oxide semiconductor film 108_1 over theinsulating film 104, an oxide semiconductor film 108_2 over the oxidesemiconductor film 108_1, and an oxide semiconductor film 108_3 over theoxide semiconductor film 108_2. The channel region 108 i, the sourceregion 108 s, and the drain region 108 d each have a three-layerstructure of the oxide semiconductor film 108_1, the oxide semiconductorfilm 108_2, and the oxide semiconductor film 108_3.

The oxide semiconductor film 108 of the transistor 100G illustrated inFIGS. 26A and 26B includes the oxide semiconductor film 108_2 over theinsulating film 104, and the oxide semiconductor film 108_3 over theoxide semiconductor film 108_2. The channel region 108 i, the sourceregion 108 s, and the drain region 108 d each have a two-layer structureof the oxide semiconductor film 108_2 and the oxide semiconductor film108_3.

The oxide semiconductor film 108 of the transistor 100H illustrated inFIGS. 27A and 27B includes the oxide semiconductor film 108_1 over theinsulating film 104, and the oxide semiconductor film 108_2 over theoxide semiconductor film 108_1. The channel region 108 i, the sourceregion 108 s, and the drain region 108 d each have a two-layer structureof the oxide semiconductor film 108_1 and the oxide semiconductor film108_2.

The oxide semiconductor film 108 of the transistor 100J illustrated inFIGS. 28A and 28B includes the oxide semiconductor film 108_1 over theinsulating film 104, the oxide semiconductor film 108_2 over the oxidesemiconductor film 108_1, and the oxide semiconductor film 108_3 overthe oxide semiconductor film 108_2. The channel region 108 i has athree-layer structure of the oxide semiconductor film 108_1, the oxidesemiconductor film 108_2, and the oxide semiconductor film 108_3. Thesource region 108 s and the drain region 108 d each have a two-layerstructure of the oxide semiconductor film 108_1 and the oxidesemiconductor film 108_2. Note that in the cross section of thetransistor 100J in the channel width (W) direction, the oxidesemiconductor film 108_3 covers side surfaces of the oxide semiconductorfilm 108_1 and the oxide semiconductor film 108_2.

The oxide semiconductor film 108 of the transistor 100K illustrated inFIGS. 29A and 29B includes the oxide semiconductor film 108_2 over theinsulating film 104, and the oxide semiconductor film 108_3 over theoxide semiconductor film 108_2. The channel region 108 i has a two-layerstructure of the oxide semiconductor film 108_2 and the oxidesemiconductor film 108_3. The source region 108 s and the drain region108 d each have a single-layer structure of the oxide semiconductor film108_2. Note that in the cross section of the transistor 100K in thechannel width (W) direction, the oxide semiconductor film 108_3 coversside surfaces of the oxide semiconductor film 108_2.

A side surface of the channel region 108 i in the channel width (W)direction or a region in the vicinity of the side surface is easilydamaged by processing, resulting in a defect (e.g., oxygen vacancy), oreasily contaminated by an impurity attached thereto. Therefore, evenwhen the channel region 108 i is substantially intrinsic, stress such asan electric field applied thereto activates the side surface of thechannel region 108 i in the channel width (W) direction or the region inthe vicinity of the side surface and turns it into a low-resistance(n-type) region easily. Moreover, if the side surface of the channelregion 108 i in the channel width (W) direction or the region in thevicinity of the side surface is an n-type region, a parasitic channelmay be formed because the n-type region serves as a carrier path.

Thus, in the transistor 100J and the transistor 100K, the channel region108 i has a stacked-layer structure and side surfaces of the channelregion 108 i in the channel width (W) direction are covered with onelayer of the stacked layers. With such a structure, defects on or in thevicinity of the side surfaces of the channel region 108 i can besuppressed or adhesion of an impurity to the side surfaces of thechannel region 108 i or to regions in the vicinity of the side surfacescan be reduced.

<2-5. Band Structure>

Here, a band structure of the insulating film 104, the oxidesemiconductor films 108_1, 108_2, and 108_3, and the insulating film110, a band structure of the insulating film 104, the oxidesemiconductor films 108_2 and 108_3, and the insulating film 110, and aband structure of the insulating film 104, the oxide semiconductor films108_1 and 108_2, and the insulating film 110 will be described withreference to FIGS. 30A to 30C. Note that FIGS. 30A to 30C are each aband structure of the channel region 108 i.

FIG. 30A shows an example of a band structure in the thickness directionof a stack including the insulating film 104, the oxide semiconductorfilms 108_1, 108_2, and 108_3, and the insulating film 110. FIG. 30Bshows an example of a band structure in the thickness direction of astack including the insulating film 104, the oxide semiconductor films108_2 and 108_3, and the insulating film 110. FIG. 30C shows an exampleof a band structure in the thickness direction of a stack including theinsulating film 104, the oxide semiconductor films 108_1 and 108_2, andthe insulating film 110. For easy understanding, the band structuresshow the conduction band minimum (E_(c)) of the insulating film 104, theoxide semiconductor films 108_1, 108_2, and 108_3, and the insulatingfilm 110.

In the band structure of FIG. 30A, a silicon oxide film is used as eachof the insulating films 104 and 110, an oxide semiconductor film formedusing a metal oxide target whose atomic ratio of In to Ga and Zn is1:3:2 is used as the oxide semiconductor film 108_1, an oxidesemiconductor film formed using a metal oxide target whose atomic ratioof In to Ga and Zn is 4:2:4.1 is used as the oxide semiconductor film108_2, and an oxide semiconductor film formed using a metal oxide targetwhose atomic ratio of In to Ga and Zn is 1:3:2 is used as the oxidesemiconductor film 108_3.

In the band structure of FIG. 30B, a silicon oxide film is used as eachof the insulating films 104 and 110, an oxide semiconductor film formedusing a metal oxide target whose atomic ratio of In to Ga and Zn is4:2:4.1 is used as the oxide semiconductor film 108_2, and an oxidesemiconductor film formed using a metal oxide target whose atomic ratioof In to Ga and Zn is 1:3:2 is used as the oxide semiconductor film108_3.

In the band structure of FIG. 30C, a silicon oxide film is used as eachof the insulating films 104 and 110, an oxide semiconductor film formedusing a metal oxide target whose atomic ratio of In to Ga and Zn is1:3:2 is used as the oxide semiconductor film 108_1, and an oxidesemiconductor film formed using a metal oxide target whose atomic ratioof In to Ga and Zn is 4:2:4.1 is used as the oxide semiconductor film108_2.

As illustrated in FIG. 30A, the conduction band minimum gradually variesbetween the oxide semiconductor films 108_1, 108_2, and 108_3. Asillustrated in FIG. 30B, the conduction band minimum gradually variesbetween the oxide semiconductor films 108_2 and 108_3. As illustrated inFIG. 30C, the conduction band minimum gradually varies between the oxidesemiconductor films 108_1 and 108_2. In other words, the conduction bandminimum is continuously changed or continuously connected. To obtainsuch a band structure, there exists no impurity, which forms a defectstate such as a trap center or a recombination center, at the interfacebetween the oxide semiconductor films 108_1 and 108_2 or the interfacebetween the oxide semiconductor films 108_2 and 108_3.

To form a continuous junction between the oxide semiconductor films108_1, 108_2, and 108_3, it is necessary to form the films successivelywithout exposure to the air with a multi-chamber deposition apparatus(sputtering apparatus) provided with a load lock chamber.

With the band structure of FIG. 30A, FIG. 30B, or FIG. 30C, the oxidesemiconductor film 108_2 serves as a well, and a channel region isformed in the oxide semiconductor film 108_2 in the transistor with thestacked-layer structure.

By providing the oxide semiconductor films 108_1 and 108_3, the oxidesemiconductor film 108_2 can be distanced away from trap states.

In addition, the trap states might be more distant from the vacuum levelthan the conduction band minimum (E_(c)) of the oxide semiconductor film108_2 functioning as a channel region, so that electrons are likely tobe accumulated in the trap states. When the electrons are accumulated inthe trap states, the electrons become negative fixed electric charge, sothat the threshold voltage of the transistor is shifted in the positivedirection. Therefore, it is preferable that the trap states be closer tothe vacuum level than the conduction band minimum (E_(c)) of the oxidesemiconductor film 108_2. Such a structure inhibits accumulation ofelectrons in the trap states. As a result, the on-state current and thefield-effect mobility of the transistor can be increased.

The conduction band minimum of each of the oxide semiconductor films108_1 and 108_3 is closer to the vacuum level than that of the oxidesemiconductor film 108_2. A typical difference between the conductionband minimum of the oxide semiconductor film 108_2 and the conductionband minimum of each of the oxide semiconductor films 108_1 and 108_3 is0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eV or less. Thatis, the difference between the electron affinity of each of the oxidesemiconductor films 108_1 and 108_3 and the electron affinity of theoxide semiconductor film 108_2 is 0.15 eV or more or 0.5 eV or more and2 eV or less or 1 eV or less.

In such a structure, the oxide semiconductor film 108_2 serves as a mainpath of a current. In other words, the oxide semiconductor film 108_2serves as a channel region, and the oxide semiconductor films 108_1 and108_3 serve as oxide insulating films. It is preferable that the oxidesemiconductor films 108_1 and 108_3 each include one or more metalelements constituting a part of the oxide semiconductor film 108_2 inwhich a channel region is formed. With such a structure, interfacescattering hardly occurs at the interface between the oxidesemiconductor film 108_1 and the oxide semiconductor film 108_2 or atthe interface between the oxide semiconductor film 108_2 and the oxidesemiconductor film 108_3. Thus, the transistor can have highfield-effect mobility because the movement of carriers is not hinderedat the interface.

To prevent each of the oxide semiconductor films 108_1 and 108_3 fromfunctioning as part of a channel region, a material having sufficientlylow conductivity is used for the oxide semiconductor films 108_1 and108_3. Thus, the oxide semiconductor films 108_1 and 108_3 can bereferred to as oxide insulating films for such properties and/orfunctions. Alternatively, a material that has a smaller electronaffinity (a difference between the vacuum level and the conduction bandminimum) than the oxide semiconductor film 108_2 and has a difference inthe conduction band minimum from the oxide semiconductor film 108_2(band offset) is used for the oxide semiconductor films 108_1 and 108_3.Furthermore, to inhibit generation of a difference in threshold voltagedue to the value of the drain voltage, it is preferable to form theoxide semiconductor films 108_1 and 108_3 using a material whoseconduction band minimum is closer to the vacuum level than that of theoxide semiconductor film 108_2. For example, a difference between theconduction band minimum of the oxide semiconductor film 108_2 and theconduction band minimum of each of the oxide semiconductor films 108_1and 108_3 is preferably greater than or equal to 0.2 eV, more preferablygreater than or equal to 0.5 eV.

It is preferable that the oxide semiconductor films 108_1 and 108_3 nothave a spinel crystal structure. This is because if the oxidesemiconductor films 108_1 and 108_3 have a spinel crystal structure,constituent elements of the conductive films 120 a and 120 b might bediffused into the oxide semiconductor film 108_2 at the interfacebetween the spinel crystal structure and another region. Note that eachof the oxide semiconductor films 108_1 and 108_3 is preferably a CAAC-OSfilm described later, in which case a higher blocking property againstconstituent elements of the conductive films 120 a and 120 b, forexample, copper elements, can be obtained.

Although the example where an oxide semiconductor film formed using ametal oxide target whose atomic ratio of In to Ga and Zn is 1:3:2, isused as each of the oxide semiconductor films 108_1 and 108_3 isdescribed in this embodiment, one embodiment of the present invention isnot limited thereto. For example, an oxide semiconductor film formedusing a metal oxide target whose atomic ratio of In to Ga and Zn is1:1:1, 1:1:1.2, 1:3:4, 1:3:6, 1:4:5, 1:5:6, or 1:10:1 may be used aseach of the oxide semiconductor films 108_1 and 108_3. Alternatively,oxide semiconductor films formed using a metal oxide target whose atomicratio of Ga to Zn is 10:1 may be used as the oxide semiconductor films108_1 and 108_3. In that case, it is suitable that an oxidesemiconductor film formed using a metal oxide target whose atomic ratioof In to Ga and Zn is 1:1:1 is used as the oxide semiconductor film108_2 because the difference between the conduction band minimum of theoxide semiconductor film 108_2 and the conduction band minimum of theoxide semiconductor film 108_1 or 108_3 can be 0.6 eV or more.

When the oxide semiconductor films 108_1 and 108_3 are formed using ametal oxide target whose atomic ratio of In to Ga and Zn is 1:1:1, theatomic ratio of In to Ga and Zn in the oxide semiconductor films 108_1and 108_3 might be 1:β1:β2 (0<β1≦2, 0<β2≦2). When the oxidesemiconductor films 108_1 and 108_3 are formed using a metal oxidetarget whose atomic ratio of In to Ga and Zn is 1:3:4, the atomic ratioof In to Ga and Zn in the oxide semiconductor films 108_1 and 108_3might be 1:β3:β4 (1≦β3≦5, 2≦β4≦6). When the oxide semiconductor films108_1 and 108_3 are formed using a metal oxide target whose atomic ratioof In to Ga and Zn is 1:3:6, the atomic ratio of In to Ga and Zn in theoxide semiconductor films 108_1 and 108_3 might be 1:β5:β6 (1≦β5≦5,4≦β6≦8).

<2-6. Method 1 For Manufacturing Transistor>

Next, an example of the method for manufacturing the transistor 100illustrated in FIGS. 19A to 19C will be described with reference toFIGS. 31A to 31D, FIGS. 32A to 32C, and FIGS. 33A and 33B. Note thatFIGS. 31A to 31D, FIGS. 32A to 32C, and FIGS. 33A and 33B arecross-sectional views in the channel length (L) direction and thechannel width (W) direction and illustrate a method for manufacturingthe transistor 100.

First, the insulating film 104 is formed over the substrate 102.Subsequently, an oxide semiconductor film is formed over the insulatingfilm 104. Then, the oxide semiconductor film is processed into an islandshape, whereby an oxide semiconductor film 107 is formed (see FIG. 31A).

The insulating film 104 can be formed by a sputtering method, a CVDmethod, an evaporation method, a pulsed laser deposition (PLD) method, aprinting method, a coating method, or the like as appropriate. In thisembodiment, as the insulating film 104, a 400-nm-thick silicon nitridefilm and a 50-nm-thick silicon oxynitride film are formed with a plasmaCVD apparatus. Note that the oxide semiconductor film 108 may be formedover the substrate 102 without forming the insulating film 104.

After the insulating film 104 is formed, oxygen may be added to theinsulating film 104. As oxygen added to the insulating film 104, anoxygen radical, an oxygen atom, an oxygen atomic ion, an oxygenmolecular ion, or the like may be used. Oxygen can be added by an iondoping method, an ion implantation method, a plasma treatment method, orthe like. Alternatively, a film that suppresses oxygen release may beformed over the insulating film 104, and then, oxygen may be added tothe insulating film 104 through the film.

The film that suppresses oxygen release can be formed using a conductivefilm or a semiconductor film containing one or more of indium, zinc,gallium, tin, aluminum, chromium, tantalum, titanium, molybdenum,nickel, iron, cobalt, and tungsten.

In the case where oxygen is added by plasma treatment in which oxygen isexcited by a microwave to generate high-density oxygen plasma, theamount of oxygen added to the insulating film 104 can be increased.

The oxide semiconductor film 107 can be formed by a sputtering method, acoating method, a pulsed laser deposition method, a laser ablationmethod, a thermal CVD method, or the like. Note that the oxidesemiconductor film can be processed into the oxide semiconductor film107 in the following manner: a mask is formed over the oxidesemiconductor film by a lithography process, and then, the oxidesemiconductor film is partly etched using the mask. Alternatively, theisolated oxide semiconductor film 107 may be directly formed by aprinting method.

As a power supply device for generating plasma when the oxidesemiconductor film is formed by a sputtering method, an RF power supplydevice, an AC power supply device, a DC power supply device, or the likecan be used as appropriate. As a sputtering gas for forming the oxidesemiconductor film, a rare gas (typically argon), oxygen, or a mixed gasof a rare gas and oxygen is used as appropriate. In the mixed gas of arare gas and oxygen, the proportion of oxygen to the rare gas ispreferably increased.

To increase the crystallinity of the oxide semiconductor film formed bya sputtering method, for example, the oxide semiconductor film ispreferably deposited at a substrate temperature higher than or equal to150° C. and lower than or equal to 750° C., higher than or equal to 150°C. and lower than or equal to 450° C., or higher than or equal to 200°C. and lower than or equal to 350° C.

In this embodiment, as the oxide semiconductor film 107, a 35-nm-thickoxide semiconductor film is deposited with a sputtering apparatus usingan In—Ga—Zn metal oxide (In:Ga:Zn=4:2:4.1 [atomic ratio]) as asputtering target.

After the oxide semiconductor film 107 is formed, the oxidesemiconductor film 107 may be dehydrated or dehydrogenated by heattreatment. The temperature of the heat treatment is typically higherthan or equal to 150° C. and lower than the strain point of thesubstrate, higher than or equal to 250° C. and lower than or equal to450° C., or higher than or equal to 300° C. and lower than or equal to450° C.

The heat treatment can be performed in an inert gas atmospherecontaining nitrogen or a rare gas such as helium, neon, argon, xenon, orkrypton. Alternatively, the heat treatment may be performed in an inertgas atmosphere first, and then, in an oxygen atmosphere. It ispreferable that the above inert gas atmosphere and the above oxygenatmosphere do not contain hydrogen, water, and the like. The treatmenttime may be longer than or equal to 3 minutes and shorter than or equalto 24 hours.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature higher than or equal to the strain pointof the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

By depositing the oxide semiconductor film while it is heated or byperforming heat treatment after the formation of the oxide semiconductorfilm, the hydrogen concentration in the oxide semiconductor film, whichis measured by SIMS, can be 5×10¹⁹ atoms/cm³ or lower, 1×10¹⁹ atoms/cm³or lower, 5×10¹⁸ atoms/cm³ or lower, 1×10¹⁸ atoms/cm³ or lower, 5×10¹⁷atoms/cm³ or lower, or 1×10¹⁶ atoms/cm³ or lower.

Next, an insulating film 110_0 is formed over the insulating film 104and the oxide semiconductor film 107 (see FIG. 31B).

For the insulating film 110_0, a silicon oxide film or a siliconoxynitride film can be formed with a plasma-enhanced chemical vapordeposition apparatus (a PECVD apparatus or simply referred to as aplasma CVD apparatus). In this case, a deposition gas containing siliconand an oxidizing gas are preferably used as a source gas. Typicalexamples of the deposition gas containing silicon include silane,disilane, trisilane, and silane fluoride. As examples of the oxidizinggas, oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide can begiven.

A silicon oxynitride film having few defects can be formed as theinsulating film 110_0 with the plasma CVD apparatus under the conditionsthat the flow rate of the oxidizing gas is more than 20 times and lessthan 100 times, or more than or equal to 40 times and less than or equalto 80 times the flow rate of the deposition gas and that the pressure ina treatment chamber is lower than 100 Pa or lower than or equal to 50Pa.

As the insulating film 110_0, a dense silicon oxide film or a densesilicon oxynitride film can be formed under the following conditions:the substrate placed in a vacuum-evacuated treatment chamber of theplasma CVD apparatus is held at a temperature higher than or equal to280° C. and lower than or equal to 400° C., the pressure in thetreatment chamber into which a source gas is introduced is set to behigher than or equal to 20 Pa and lower than or equal to 250 Pa,preferably higher than or equal to 100 Pa and lower than or equal to 250Pa, and a high-frequency power is supplied to an electrode provided inthe treatment chamber.

The insulating film 110_0 may be formed by a plasma CVD method using amicrowave. A microwave refers to a wave in the frequency range of 300MHz to 300 GHz. In a microwave, electron temperature and electron energyare low. Furthermore, in supplied power, the proportion of power usedfor acceleration of electrons is low, and therefore, much more power canbe used for dissociation and ionization of molecules. Thus, plasma witha high density (high-density plasma) can be excited. This method causeslittle plasma damage to the deposition surface or a deposit, so that theinsulating film 110_0 having few defects can be formed.

Alternatively, the insulating film 110_0 can also be formed by a CVDmethod using an organosilane gas. As the organosilane gas, the followingsilicon-containing compound can be used: tetraethyl orthosilicate (TEOS)(chemical formula: Si(OC₂H₅)₄), tetramethylsilane (TMS) (chemicalformula: Si(CH₃)₄), tetramethylcyclotetrasiloxane (TMCTS),octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDS),triethoxysilane (SiH(OC₂H₅)₃), trisdimethylaminosilane (SiH(N(CH₃)₂)₃),or the like. By a CVD method using an organosilane gas, the insulatingfilm 110_0 having high coverage can be formed.

In this embodiment, as the insulating film 110_0, a 100-nm-thick siliconoxynitride film is formed with the plasma CVD apparatus.

Next, a conductive film 112_0 is formed over the insulating film 110_0.In the case where a metal oxide film is used as the conductive film112_0, for example, oxygen might be added from the conductive film 112_0to the insulating film 110_0 during the formation of the conductive film112_0 (see FIG. 31C).

In FIG. 31C, oxygen added to the insulating film 110_0 is schematicallyshown by arrows.

In the case where a metal oxide film is used as the conductive film112_0, the conductive film 112_0 is preferably formed by a sputteringmethod in an atmosphere containing an oxygen gas. Formation of theconductive film 112_0 in an atmosphere containing an oxygen gas allowssuitable addition of oxygen to the insulating film 110_0. Note that amethod for forming the conductive film 112_0 is not limited to asputtering method, and other methods such as an ALD method may be used.

In this embodiment, a 100-nm-thick IGZO film containing an In—Ga—Znoxide (In:Ga:Zn=4:2:4.1 [atomic ratio]) is formed as the conductive film112_0 by a sputtering method. Note that oxygen addition treatment may beperformed on the insulating film 110_0 before or after the formation ofthe conductive film 112_0. The oxygen addition treatment can beperformed similarly to the oxygen addition that can be performed afterthe formation of the insulating film 104.

Subsequently, a mask 140 is formed by a lithography process in a desiredposition over the conductive film 112_0 (see FIG. 31D).

Next, etching is performed from above the mask 140 to process theconductive film 112_0 and the insulating film 110_0. Then, the mask 140is removed, so that the island-shaped conductive film 112 and theisland-shaped insulating film 110 are formed (see FIG. 32A).

In this embodiment, the conductive film 112_0 and the insulating film110_0 are processed by a dry etching method.

In the processing of the conductive film 112_0 and the insulating film110_0, the thickness of the oxide semiconductor film 107 in a region notoverlapping with the conductive film 112 is decreased in some cases. Inother cases, in the processing of the conductive film 112_0 and theinsulating film 110_0, the thickness of the insulating film 104 in aregion not overlapping with the oxide semiconductor film 107 isdecreased. In the processing of the conductive film 112_0 and theinsulating film 110_0, an etchant or an etching gas (e.g., chlorine)might be added to the oxide semiconductor film 107 or the constituentelement of the conductive film 112_0 or the insulating film 110_0 mightbe added to the oxide semiconductor film 107.

Then, the insulating film 116 is formed over the insulating film 104,the oxide semiconductor film 107, and the conductive film 112. Note thatwhen the insulating film 116 is formed, the oxide semiconductor film 107in regions in contact with the insulating film 116 becomes the sourceregion 108 s and the drain region 108 d. The oxide semiconductor film107 in a region in contact with the insulating film 110 becomes thechannel region 108 i. Accordingly, the oxide semiconductor film 108including the channel region 108 i, the source region 108 s, and thedrain region 108 d is formed (see FIG. 32B).

When a silicon nitride oxide film is used for the insulating film 116,nitrogen or hydrogen in the silicon nitride oxide film can be suppliedto the source region 108 s and the drain region 108 d in contact withthe insulating film 116.

Note that an impurity element may be added to the oxide semiconductorfilm 107 before the insulating film 116 is formed. Alternatively, animpurity element may be added to the oxide semiconductor film 107through the insulating film 116 after the insulating film 116 is formed.

The impurity element can be added by an ion doping method, an ionimplantation method, a plasma treatment method, or the like. In a plasmatreatment method, an impurity element can be added using plasmagenerated in a gas atmosphere containing the impurity element. A dryetching apparatus, an ashing apparatus, a plasma CVD apparatus, ahigh-density plasma CVD apparatus, or the like can be used to generateplasma.

As a source gas of the impurity element, at least one of B₂H₆, PH₃, CH₄,N₂, NH₃, AlH₃, AlCl₃, SiH₄, Si₂H₆, F₂, HF, H₂, and a rare gas can beused. Alternatively, at least one of B₂H₆, PH₃, N₂, NH₃, AlH₃, AlCl₃,F₂, HF, and H₂ which are diluted with a rare gas can be used. Typicalexamples of the rare gas element include helium, neon, argon, krypton,and xenon.

Alternatively, after a rare gas is added to the oxide semiconductor film107, at least one of B₂H₆, PH₃, CH₄, N₂, NH₃, AlH₃, AlCl₃, SiH₄, Si₂H₆,F₂, HF, and H₂ may be added thereto. Further alternatively, after atleast one of B₂H₆, PH₃, CH₄, N₂, NH₃, AlH₃, AlCl₃, SiH₄, Si₂H₆, F₂, HF,and H₂ is added to the oxide semiconductor film 107, a rare gas may beadded thereto.

Next, the insulating film 118 is formed over the insulating film 116(see FIG. 32C).

The insulating film 118 can be formed using a material selected from theabove-mentioned materials. In this embodiment, as the insulating film118, a 300-nm-thick silicon oxynitride film is formed with a plasma CVDapparatus.

Subsequently, a mask is formed by lithography in a desired position overthe insulating film 118, and then, the insulating film 118 and theinsulating film 116 are partly etched, so that the opening 141 areaching the source region 108 s and the opening 141 b reaching thedrain region 108 d are formed (see FIG. 33A).

To etch the insulating film 118 and the insulating film 116, a wetetching method and/or a dry etching method can be used. In thisembodiment, the insulating film 118 and the insulating film 116 areprocessed by a dry etching method.

Next, a conductive film is formed over the source region 108 s, thedrain region 108 d, and the insulating film 118 so as to cover theopenings 141 a and 141 b and the conductive film is processed into adesired shape, whereby the conductive films 120 a and 120 b are formed(see FIG. 33B).

The conductive films 120 a and 120 b can be formed using a materialselected from the above-mentioned materials. In this embodiment, for theconductive films 120 a and 120 b, a stack including a 50-nm-thicktungsten film and a 400-nm-thick copper film is formed with a sputteringapparatus.

To process the conductive film to be the conductive films 120 a and 120b, a wet etching method and/or a dry etching method can be used. In thisembodiment, in the processing of the conductive film into the conductivefilms 120 a and 120 b, the copper film is etched by a wet etching methodand then the tungsten film is etched by a dry etching method.

Through the above steps, the transistor 100 in FIGS. 19A to 19C can bemanufactured.

Note that the films constituting a part of the transistor 100 (theinsulating film, the metal oxide film, the oxide semiconductor film, theconductive film, and the like) can be formed by, other than the abovemethods, a sputtering method, a chemical vapor deposition (CVD) method,a vacuum evaporation method, a pulsed laser deposition (PLD) method, oran ALD method. Alternatively, a coating method or a printing method canbe used. Although a sputtering method and a plasma-enhanced chemicalvapor deposition (PECVD) method are typical deposition methods, athermal CVD method may also be used. As an example of a thermal CVDmethod, a metal organic chemical vapor deposition (MOCVD) method can begiven.

Deposition by a thermal CVD method is performed in the following manner:a source gas and an oxidizer are supplied at a time to a chamber inwhich the pressure is set to an atmospheric pressure or a reducedpressure, and the source gas and the oxidizer react with each other inthe vicinity of the substrate or over the substrate. As seen above, noplasma is generated during deposition by a thermal CVD method, which hasan advantage in that no defect due to plasma damage is formed.

Films such as the conductive film, the insulating film, the oxidesemiconductor film, and the metal oxide film can be formed by a thermalCVD method such as an MOCVD method. For example, in the case where anIn—Ga—Zn—O film is deposited, trimethylindium (In(CH₃)₃),trimethylgallium (Ga(CH₃)₃), and dimethylzinc (Zn(CH₃)₂) are used.Without being limited to the above combination, triethylgallium(Ga(C₂H₅)₃) can be used instead of trimethylgallium, and diethylzinc(Zn(C₂H₅)₂) can be used instead of dimethylzinc.

In the case where a hafnium oxide film is formed with a depositionapparatus employing an ALD method, two kinds of gases are used, namely,ozone (O₃) as an oxidizer and a source gas which is obtained byvaporizing liquid containing a solvent and a hafnium precursor (hafniumalkoxide or hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH,Hf[N(CH₃)₂]₄) or tetrakis(ethylmethylamide)hafnium).

In the case where an aluminum oxide film is formed with a depositionapparatus employing an ALD method, two kinds of gases are used, namely,H₂O as an oxidizer and a source gas which is obtained by vaporizingliquid containing a solvent and an aluminum precursor (e.g.,trimethylaluminum (TMA, Al(CH₃)₃)). Examples of another material includetris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

In the case where a silicon oxide film is formed with a depositionapparatus employing an ALD method, hexachlorodisilane is adsorbed on asurface on which a film is to be deposited, and radicals of an oxidizinggas (O₂ or dinitrogen monoxide) are supplied to react with theadsorbate.

In the case where a tungsten film is formed with a deposition apparatusemploying an ALD method, a WF₆ gas and a B₂H₆ gas are sequentiallyintroduced to form an initial tungsten film, and then, a WF₆ gas and anH₂ gas are used to form a tungsten film. Note that an SiH₄ gas may beused instead of a B₂H₆ gas.

In the case where an oxide semiconductor film such as an In—Ga—Zn—O filmis formed with a deposition apparatus employing an ALD method, anIn(CH₃)₃ gas and an O₃ gas are used to form an In—O layer, a Ga(CH₃)₃gas and an O₃ gas are used to form a Ga—O layer, and then, a Zn(CH₃)₂gas and an O₃ gas are used to form a Zn—O layer. Note that the order ofthese layers is not limited to this example. A mixed compound layer suchas an In—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formedusing these gases. Although an H₂O gas which is obtained by bubblingwater with an inert gas such as Ar may be used instead of an O₃ gas, itis preferable to use an O₃ gas, which does not contain H.

<2-7. Method 2 For Manufacturing Transistor>

Next, an example of a method for manufacturing the transistor 100A inFIGS. 20A to 20C will be described with reference to FIGS. 34A to 34D,FIGS. 35A to 35C, and FIGS. 36A to 36C. Note that FIGS. 34A to 34D,FIGS. 35A to 35C, and FIGS. 36A to 36C are cross-sectional views in thechannel length (L) direction and the channel width (W) direction andillustrate a method for manufacturing the transistor 100A.

First, the conductive film 106 is formed over the substrate 102. Then,the insulating film 104 is formed over the substrate 102 and theconductive film 106, and an oxide semiconductor film is formed over theinsulating film 104. After that, the oxide semiconductor film isprocessed into an island shape, whereby the oxide semiconductor film 107is formed (see FIG. 34A).

The conductive film 106 can be formed using a material and a methodsimilar to those of the conductive films 120 a and 120 b. In thisembodiment, as the conductive film 106, a stack including a 50-nm-thicktantalum nitride film and a 100-nm-thick copper film is formed by asputtering method.

Next, the insulating film 110_0 is formed over the insulating film 104and the oxide semiconductor film 107 (see FIG. 34B).

Subsequently, a mask is formed by lithography in a desired position overthe insulating film 110_0, and then, the insulating film 110_0, and theinsulating film 104 are partly etched, so that the opening 143 reachingthe conductive film 106 is formed (see FIG. 34C).

To form the opening 143, a wet etching method and/or a dry etchingmethod can be used. In this embodiment, the opening 143 is formed by adry etching method.

Next, the conductive film 112_0 is formed over the conductive film 106and the insulating film 110_0 so as to cover the opening 143. In thecase where a metal oxide film is used as the conductive film 112_0, forexample, oxygen might be added from the conductive film 112_0 to theinsulating film 110_0 during the formation of the conductive film 112_0(see FIG. 34D).

In FIG. 34D, oxygen added to the insulating film 110_0 is schematicallyshown by arrows. Furthermore, the conductive film 112_0 formed to coverthe opening 143 is electrically connected to the conductive film 106.

Subsequently, the mask 140 is formed by a lithography process in adesired position over the conductive film 112_0 (see FIG. 35A).

Next, etching is performed from above the mask 140 to process theconductive film 112_0 and the insulating film 110_0. After theprocessing of the conductive film 112_0 and the insulating film 110_0,the mask 140 is removed. As a result of the processing of the conductivefilm 112_0 and the insulating film 110_0, the island-shaped conductivefilm 112 and the island-shaped insulating film 110 are formed (see FIG.35B).

In this embodiment, the conductive film 112_0 and the insulating film110_0 are processed by a dry etching method.

After that, the insulating film 116 is formed over the insulating film104, the oxide semiconductor film 107, and the conductive film 112. Notethat when the insulating film 116 is formed, the oxide semiconductorfilm 107 in regions in contact with the insulating film 116 becomes thesource region 108 s and the drain region 108 d. The oxide semiconductorfilm 107 in a region in contact with the insulating film 110 becomes thechannel region 108 i. Accordingly, the oxide semiconductor film 108including the channel region 108 i, the source region 108 s, and thedrain region 108 d is formed (see FIG. 35C).

Note that the insulating film 116 can be formed using a materialselected from the above-mentioned materials. In this embodiment, as theinsulating film 116, a 100-nm-thick silicon nitride oxide film is formedwith a plasma CVD apparatus. In the formation of the silicon nitrideoxide film, plasma treatment and deposition treatment are performed at220° C. Note that the plasma treatment and the deposition treatment canbe performed in the same manner described above.

Next, the insulating film 118 is formed over the insulating film 116(see FIG. 36A).

Subsequently, a mask is formed by lithography in a desired position overthe insulating film 118, and then, the insulating film 118 and theinsulating film 116 are partly etched, so that the opening 141 areaching the source region 108 s and the opening 141 b reaching thedrain region 108 d are formed (see FIG. 36B).

Next, a conductive film is formed over the source region 108 s, thedrain region 108 d, and the insulating film 118 so as to cover theopenings 141 a and 141 b and the conductive film is processed into adesired shape, whereby the conductive films 120 a and 120 b are formed(see FIG. 36C).

Through the above steps, the transistor 100A in FIGS. 20A to 20C can bemanufactured.

One embodiment of the present invention is not limited to the exampledescribed in this embodiment, in which the transistor includes an oxidesemiconductor film. In one embodiment of the present invention, thetransistor does not necessarily include an oxide semiconductor film. Forexample, a channel region, the vicinity of the channel region, a sourceregion, or a drain region of the transistor may be formed using amaterial containing silicon (Si), germanium (Ge), silicon germanium(SiGe), gallium arsenide (GaAs), or the like.

The structures and the methods described in this embodiment can becombined as appropriate with any of the structures and the methodsdescribed in the other embodiments.

Embodiment 3

In this embodiment, the structure and the like of an oxide semiconductorwill be described with reference to FIGS. 37A to 37E, FIGS. 38A to 38E,FIGS. 39A to 39D, FIGS. 40A and 40B, and FIG. 41.

<3-1. Structure of Oxide Semiconductor>

An oxide semiconductor is classified into a single-crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofthe non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of the crystalline oxide semiconductor include a single-crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

An amorphous structure is generally thought to be isotropic and have nonon-uniform structure, to be metastable and have no fixed atomicarrangement, to have a flexible bond angle, and to have a short-rangeorder but have no long-range order, for example.

In other words, a stable oxide semiconductor cannot be regarded as acompletely amorphous oxide semiconductor. Moreover, an oxidesemiconductor that is not isotropic (e.g., an oxide semiconductor thathas a periodic structure in a microscopic region) cannot be regarded asa completely amorphous oxide semiconductor. In contrast, an a-like OS,which is not isotropic, has an unstable structure that contains a void.Because of its instability, an a-like OS has physical properties similarto those of an amorphous oxide semiconductor.

<3-2. CAAC-OS>

First, a CAAC-OS will be described.

A CAAC-OS is one of oxide semiconductors and has a plurality of c-axisaligned crystal parts (also referred to as pellets).

Analysis of a CAAC-OS by X-ray diffraction (XRD) will be described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystal,which is classified into the space group R-3m, is analyzed by anout-of-plane method, a peak appears at a diffraction angle (2θ) ofaround 31° as shown in FIG. 37A. This peak is derived from the (009)plane of the InGaZnO₄ crystal, which indicates that crystals in theCAAC-OS have c-axis alignment and that the c-axes are aligned in thedirection substantially perpendicular to a surface over which theCAAC-OS is formed (also referred to as a formation surface) or a topsurface of the CAAC-OS. Note that a peak sometimes appears at 2θ ofaround 36° in addition to the peak at 2θ of around 31°. The peak at 2θof around 36° is attributed to a crystal structure classified into thespace group Fd-3m; thus, this peak is preferably not exhibited in theCAAC-OS.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on the CAAC-OS in the directionparallel to the formation surface, a peak appears at 2θ of around 56°.This peak is derived from the (110) plane of the InGaZnO₄ crystal. Whenanalysis (φ scan) is performed with 2θ fixed at around 56° while thesample is rotated around a normal vector to the sample surface as anaxis (φ axis), as shown in FIG. 37B, a peak is not clearly observed. Incontrast, in the case where single-crystal InGaZnO₄ is subjected to φscan with 2θ fixed at around 56°, as shown in FIG. 37C, six peaks whichare derived from crystal planes equivalent to the (110) plane areobserved. Accordingly, the structural analysis using XRD shows that thedirections of the a-axes and b-axes are irregularly oriented in theCAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction will be described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in the directionparallel to the formation surface of the CAAC-OS, a diffraction pattern(also referred to as a selected-area electron diffraction pattern) inFIG. 37D can be obtained. This diffraction pattern includes spotsderived from the (009) plane of the InGaZnO₄ crystal. Thus, the resultsof electron diffraction also indicate that pellets included in theCAAC-OS have c-axis alignment and that the c-axes are aligned in thedirection substantially perpendicular to the formation surface or thetop surface of the CAAC-OS. Meanwhile, FIG. 37E shows a diffractionpattern obtained in such a manner that an electron beam with a probediameter of 300 nm is incident on the same sample in the directionperpendicular to the sample surface. In FIG. 37E, a ring-likediffraction pattern is observed. Thus, the results of electrondiffraction using an electron beam with a probe diameter of 300 nm alsoindicate that the a-axes and b-axes of the pellets included in theCAAC-OS do not have regular alignment. The first ring in FIG. 37E isderived from the (010) plane, the (100) plane, and the like of theInGaZnO₄ crystal. The second ring in FIG. 37E is derived from the (110)plane and the like.

In a combined analysis image (also referred to as a high-resolutiontransmission electron microscope (TEM) image) of a bright-field imageand a diffraction pattern of a CAAC-OS, which is obtained using a TEM, aplurality of pellets can be observed. However, even in thehigh-resolution TEM image, a boundary between pellets, that is, a grainboundary is not clearly observed in some cases. Thus, in the CAAC-OS, areduction in electron mobility due to the grain boundary is less likelyto occur.

FIG. 38A shows a high-resolution TEM image of a cross section of theCAAC-OS which is observed in the direction substantially parallel to thesample surface. The high-resolution TEM image is obtained with aspherical aberration corrector function. The high-resolution TEM imageobtained with a spherical aberration corrector function is particularlyreferred to as a Cs-corrected high-resolution TEM image. TheCs-corrected high-resolution TEM image can be observed with, forexample, an atomic resolution analytical electron microscope JEM-ARM200Fmanufactured by JEOL Ltd.

FIG. 38A shows pellets in which metal atoms are arranged in a layeredmanner. FIG. 38A proves that the size of a pellet is greater than orequal to 1 nm or greater than or equal to 3 nm. Therefore, the pelletcan also be referred to as a nanocrystal (nc). Furthermore, the CAAC-OScan also be referred to as an oxide semiconductor including c-axisaligned nanocrystals (CANC). A pellet reflects unevenness of a formationsurface or a top surface of the CAAC-OS and is parallel to the formationsurface or the top surface of the CAAC-OS.

FIGS. 38B and 38C show Cs-corrected high-resolution TEM images of aplane of the CAAC-OS observed in the direction substantiallyperpendicular to the sample surface. FIGS. 38D and 38E are imagesobtained by image processing of FIGS. 38B and 38C. The method of imageprocessing is as follows. The image in FIG. 38B is subjected to fastFourier transform (FFT) to obtain an FFT image. Then, mask processing isperformed on the obtained FFT image such that part in the range of 2.8nm⁻¹ to 5.0 nm⁻¹ from the reference point is left. After the maskprocessing, the FFT image is subjected to inverse fast Fourier transform(IFFT) to obtain a processed image. The image obtained in this manner isreferred to as an FFT filtering image. The FFT filtering image is aCs-corrected high-resolution TEM image from which a periodic componentis extracted and shows a lattice arrangement.

In FIG. 38D, a portion in which the lattice arrangement is broken isshown by dashed lines. A region surrounded by dashed lines correspondsto one pellet. The portion denoted by the dashed lines is a junction ofpellets. The dashed lines draw a hexagon, which means that the pellethas a hexagonal shape. Note that the shape of the pellet is not always aregular hexagon but is a non-regular hexagon in many cases.

In FIG. 38E, a dotted line denotes a portion between a region where alattice arrangement is well aligned and another region where a latticearrangement is well aligned, and dashed lines denote the directions ofthe lattice arrangements. A clear crystal grain boundary cannot beobserved even in the vicinity of the dotted line. When a lattice pointin the vicinity of the dotted line is regarded as a center andsurrounding lattice points are joined, a distorted hexagon, a distortedpentagon, or a distorted heptagon can be formed, for example. That is, alattice arrangement is distorted so that formation of a crystal grainboundary is inhibited. This is probably because the CAAC-OS can toleratedistortion owing to a low density of the atomic arrangement in an a-bplane direction, the interatomic bond distance changed by substitutionof a metal element, and the like.

As described above, the CAAC-OS has c-axis alignment, its pellets(nanocrystals) are connected in the a-b plane direction, and its crystalstructure has distortion. For this reason, the CAAC-OS can also bereferred to as an oxide semiconductor including a c-axis-aligneda-b-plane-anchored (CAA) crystal.

The CAAC-OS is an oxide semiconductor with high crystallinity. Entry ofimpurities, formation of defects, or the like might decrease thecrystallinity of an oxide semiconductor. This means that the CAAC-OS hasfew impurities and defects (e.g., oxygen vacancies).

Note that an impurity means an element other than the main components ofan oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (e.g., silicon) havingstronger bonding force to oxygen than a metal element constituting apart of an oxide semiconductor extracts oxygen from the oxidesemiconductor, which results in a disordered atomic arrangement andreduced crystallinity of the oxide semiconductor. A heavy metal such asiron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. Impuritiescontained in the oxide semiconductor might serve as carrier traps orcarrier generation sources, for example. For example, an oxygen vacancyin the oxide semiconductor might serve as a carrier trap or serve as acarrier generation source when hydrogen is captured therein.

The CAAC-OS having few impurities and oxygen vacancies is an oxidesemiconductor with a low carrier density (specifically, lower than8×10¹¹ cm⁻³, preferably lower than 1×10¹¹ cm⁻³, further preferably lowerthan 1×10¹⁰ cm⁻³, and higher than or equal to 1×10⁻⁹ cm⁻³). Such anoxide semiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.Thus, the CAAC-OS can be regarded as an oxide semiconductor havingstable characteristics.

<3-3. nc-OS>

Next, an nc-OS will be described.

Analysis of an nc-OS by XRD will be described. When the structure of annc-OS is analyzed by an out-of-plane method, a peak indicatingorientation does not appear. That is, a crystal of an nc-OS does nothave orientation.

For example, when an electron beam with a probe diameter of 50 nm isincident on a 34-nm-thick region of a thinned nc-OS including anInGaZnO₄ crystal in the direction parallel to the formation surface, aring-like diffraction pattern (nanobeam electron diffraction pattern)shown in FIG. 39A is observed. FIG. 39B shows a diffraction pattern(nanobeam electron diffraction pattern) obtained when an electron beamwith a probe diameter of 1 nm is incident on the same sample. In FIG.39B, a plurality of spots are observed in a ring-like region. Thus,ordering in an nc-OS is not observed with an electron beam with a probediameter of 50 nm but is observed with an electron beam with a probediameter of 1 nm.

When an electron beam with a probe diameter of 1 nm is incident on aregion with a thickness less than 10 nm, an electron diffraction patternin which spots are arranged in an approximately regular hexagonal shapeas shown in FIG. 39C is observed in some cases. This means that an nc-OShas a well-ordered region, that is, a crystal, in the thickness range ofless than 10 nm. Note that an electron diffraction pattern havingregularity is not observed in some regions because crystals are alignedin various directions.

FIG. 39D shows a Cs-corrected high-resolution TEM image of a crosssection of an nc-OS observed in the direction substantially parallel tothe formation surface. In the high-resolution TEM image, the nc-OS has aregion in which a crystal part is observed as indicated by additionallines and a region in which a crystal part is not clearly observed. Inmost cases, the size of a crystal part included in the nc-OS is greaterthan or equal to 1 nm and less than or equal to 10 nm, specificallygreater than or equal to 1 nm and less than or equal to 3 nm. Note thatan oxide semiconductor including a crystal part whose size is greaterthan 10 nm and less than or equal to 100 nm may be referred to as amicrocrystalline oxide semiconductor. In a high-resolution TEM image ofthe nc-OS, for example, a grain boundary is not clearly observed in somecases. Note that there is a possibility that the origin of thenanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, acrystal part of the nc-OS may be referred to as a pellet in thefollowing description.

As described above, in the nc-OS, a microscopic region (for example, aregion with a size greater than or equal to 1 nm and less than or equalto 10 nm, in particular, a region with a size greater than or equal to 1nm and less than or equal to 3 nm) has a periodic atomic arrangement.There is no regularity of crystal orientation between different pelletsin the nc-OS. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS cannot be distinguished from ana-like OS or an amorphous oxide semiconductor, depending on an analysismethod.

Since there is no regularity of crystal orientation between the pellets(nanocrystals), the nc-OS can also be referred to as an oxidesemiconductor including random aligned nanocrystals (RANC) or an oxidesemiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has higher regularity than anamorphous oxide semiconductor. Therefore, the nc-OS has a lower densityof defect states than the a-like OS and the amorphous oxidesemiconductor. Note that there is no regularity of crystal orientationbetween different pellets in the nc-OS. Therefore, the nc-OS has ahigher density of defect states than the CAAC-OS.

<3-4. a-like OS>

An a-like OS has a structure between the structure of an nc-OS and thestructure of an amorphous oxide semiconductor.

FIGS. 40A and 40B show high-resolution cross-sectional TEM images of ana-like OS. The high-resolution cross-sectional TEM image of the a-likeOS in FIG. 40A is taken at the start of the electron irradiation. Thehigh-resolution cross-sectional TEM image of the a-like OS in FIG. 40Bis taken after the irradiation with electrons (e⁻) at 4.3×10⁸ e⁻/nm².FIGS. 40A and 40B show that striped bright regions extending verticallyare observed in the a-like OS from the start of the electronirradiation. It can be also found that the shape of the bright regionchanges after the electron irradiation. Note that the bright region ispresumably a void or a low-density region.

The a-like OS has an unstable structure because it contains a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation will be described below.

An a-like OS, an nc-OS, and a CAAC-OS are prepared as samples. Each ofthe samples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

It is known that a unit cell of an InGaZnO₄ crystal has a structure inwhich nine layers including three In—O layers and six Ga—Zn—O layers arestacked in the c-axis direction. The distance between the adjacentlayers is equivalent to the lattice spacing on the (009) plane (alsoreferred to as d value). The value is calculated to be 0.29 nm fromcrystal structural analysis. Accordingly, a portion in which the spacingbetween lattice fringes is greater than or equal to 0.28 nm and lessthan or equal to 0.30 nm is regarded as a crystal part of InGaZnO₄ inthe following description. Each lattice fringe corresponds to the a-bplane of the InGaZnO₄ crystal.

FIG. 41 shows a change in the average size of crystal parts (at 22points to 30 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 41 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose in obtaining TEM images, for example. As shownin FIG. 41, a crystal part with a size of approximately 1.2 nm (alsoreferred to as an initial nucleus) at the start of TEM observation growsto a size of approximately 1.9 nm at a cumulative electron (e⁻) dose of4.2×10⁸ e⁻/nm². In contrast, the crystal part sizes in the nc-OS and theCAAC-OS show few changes from the start of electron irradiation to acumulative electron dose of 4.2×10⁸ e⁻/nm². As shown in FIG. 41, thecrystal part sizes in the nc-OS and the CAAC-OS are approximately 1.3 nmand approximately 1.8 nm, respectively, regardless of the cumulativeelectron dose. For the electron beam irradiation and TEM observation, aHitachi H-9000NAR transmission electron microscope was used. Theconditions of the electron beam irradiation were as follows: theaccelerating voltage was 300 kV; the current density was 6.7×10⁵e⁻/(nm²·s); and the diameter of an irradiation region was 230 nm.

In this manner, growth of the crystal part in the a-like OS may beinduced by electron irradiation. In contrast, in the nc-OS and theCAAC-OS, growth of the crystal part is hardly induced by electronirradiation. That is, the a-like OS has an unstable structure ascompared with the nc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit contains a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of thesingle-crystal oxide semiconductor having the same composition. Thedensity of the nc-OS and the density of the CAAC-OS are each higher thanor equal to 92.3% and lower than 100% of the density of thesingle-crystal oxide semiconductor having the same composition. It isdifficult to deposit an oxide semiconductor having a density lower than78% of the density of the single-crystal oxide semiconductor.

For example, in the case of an oxide semiconductor whose atomic ratio ofIn to Ga and Zn is 1:1:1, the density of single-crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor whose atomic ratio of In to Ga and Zn is1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm³and lower than 5.9 g/cm³, for example. In the case of the oxidesemiconductor whose atomic ratio of In to Ga and Zn is 1:1:1, thedensity of the nc-OS and the density of the CAAC-OS are each higher thanor equal to 5.9 g/cm³ and lower than 6.3 g/cm³, for example.

In the case where an oxide semiconductor having a certain compositiondoes not exist in a single-crystal state, single-crystal oxidesemiconductors with different compositions are combined at an adequateratio, which makes it possible to calculate a density equivalent to thatof a single-crystal oxide semiconductor with the desired composition.The density of a single-crystal oxide semiconductor having the desiredcomposition may be calculated using a weighted average with respect tothe combination ratio of the single-crystal oxide semiconductors withdifferent compositions. Note that it is preferable to use as few kindsof single-crystal oxide semiconductors as possible to calculate thedensity.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedfilm including two or more of an amorphous oxide semiconductor, ana-like OS, an nc-OS, and a CAAC-OS, for example.

The structures described in this embodiment can be used in appropriatecombination with the structures described in any of the otherembodiments.

Embodiment 4

In this embodiment, a display module and electronic devices that includethe display device of one embodiment of the present invention will bedescribed with reference to FIG. 42, FIGS. 43A to 43E, and FIGS. 44A to44E, and FIGS. 45A and 45B.

<4-1. Display Module>

In a display module 8000 illustrated in FIG. 42, a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a frame 8009, a printed board 8010, and a battery 8011 are providedbetween an upper cover 8001 and a lower cover 8002.

The display device of one embodiment of the present invention can beused for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and may overlap with the display panel 8006. Alternatively,a counter substrate (sealing substrate) of the display panel 8006 canhave a touch panel function. Alternatively, a photosensor may beprovided in each pixel of the display panel 8006 so as to function as anoptical touch panel.

The frame 8009 protects the display panel 8006 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 can function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<4-2. Electronic Device>

FIGS. 43A to 43E and FIGS. 44A to 44E illustrate electronic devices.These electronic devices can include a housing 9000, a display portion9001, a camera 9002, a speaker 9003, an operation key 9005 (including apower switch or an operation switch), a connection terminal 9006, asensor 9007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared ray), a microphone 9008, and the like.

The electronic devices illustrated in FIGS. 43A to 43E and FIGS. 44A to44 E can have a variety of functions, for example, a function ofdisplaying a variety of data (a still image, a moving image, a textimage, and the like) on the display portion, a touch panel function, afunction of displaying a calendar, date, time, and the like, a functionof controlling a process with a variety of software (programs), awireless communication function, a function of being connected to avariety of computer networks with a wireless communication function, afunction of transmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Note that functions of the electronic devicesillustrated in FIGS. 43A to 43E and FIGS. 44A to 44E are not limitedthereto, and the electronic devices may have other functions.

The electronic devices illustrated in FIGS. 43A to 43E and FIGS. 44A to44E will be described in detail below.

FIG. 43A is a perspective view illustrating a television device 9100.The television device 9100 can include the display portion 9001 having alarge screen size of, for example, 50 inches or more, 80 inches or more,or 100 inches or more.

FIG. 43B, FIG. 43C, FIG. 43D, and FIG. 43E are perspective viewsillustrating a portable information terminal 9101, a portableinformation terminal 9102, a portable information terminal 9103, and aportable information terminal 9104, respectively.

The portable information terminal 9101 illustrated in FIG. 43B has, forexample, one or more of a function of a telephone set, a notebook, andan information browsing system. Specifically, the portable informationterminal 9101 can be used as a smartphone. Although not illustrated, thespeaker 9003, the connection terminal 9006, the sensor 9007, and thelike may be provided in the portable information terminal 9101. Theportable information terminal 9101 can display characters and imageinformation on its plurality of surfaces. For example, three operationbuttons 9050 (also referred to as operation icons or simply icons) canbe displayed on one surface of the display portion 9001. Furthermore,information 9051 indicated by dashed rectangles can be displayed onanother surface (for example, a side surface) of the display portion9001. Examples of the information 9051 include notification from asocial networking service (SNS), display indicating reception of ane-mail or an incoming call, the title of the e-mail, the SNS, or thelike, the sender of the e-mail, the SNS, or the like, the date, thetime, remaining battery, and the strength of a received signal.Alternatively, the operation buttons 9050 or the like may be displayedin place of the information 9051. The display portion 9001 of theportable information terminal 9101 partly has a curved surface.

The portable information terminal 9102 illustrated in FIG. 43C has afunction of displaying information, for example, on three or more sidesof the display portion 9001. Here, information 9052, information 9053,and information 9054 are displayed on different sides. For example, auser of the portable information terminal 9102 can see the display(here, the information 9053) with the portable information terminal 9102put in a breast pocket of his/her clothes. Specifically, a caller'sphone number, name, or the like of an incoming call is displayed in aposition that can be seen from above the portable information terminal9102. Thus, the user can see the display without taking out the portableinformation terminal 9102 from the pocket and decide whether to answerthe call. The display portion 9001 of the portable information terminal9102 partly has a curved surface.

Unlike in the portable information terminals 9101 and 9102 describedabove, the display portion 9001 does not have a curved surface in theportable information terminal 9103 illustrated in FIG. 43D.

The display portion 9001 of the portable information terminals 9104illustrated in FIG. 43E is curved. As illustrated in FIG. 43E, it ispreferable that the portable information terminal 9104 be provided witha camera 9002 to have a function of taking a still image, a function oftaking a moving image, a function of storing the taken image in a memorymedium (an external memory medium or a memory medium incorporated in thecamera), a function of displaying the taken image on the display portion9001, or the like.

FIG. 44A is a perspective view of a watch-type portable informationterminal 9200. FIG. 44B is a perspective view of a watch-type portableinformation terminal 9201.

The portable information terminal 9200 illustrated in FIG. 44A iscapable of executing a variety of applications such as mobile phonecalls, e-mailing, viewing and editing texts, music reproduction,Internet communication, and computer games. The display surface of thedisplay portion 9001 is bent, and images can be displayed on the bentdisplay surface. The portable information terminal 9200 can employ nearfield communication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication can be performed, and thus hands-free calling ispossible. Moreover, the portable information terminal 9200 includes theconnection terminal 9006, and data can be directly transmitted to andreceived from another information terminal via a connector. Chargingthrough the connection terminal 9006 is possible. Note that the chargingoperation may be performed by wireless power feeding without using theconnection terminal 9006.

Unlike in the portable information terminal 9200 illustrated in FIG.44A, the display surface of the display portion 9001 is not curved inthe portable information terminal 9201 illustrated in FIG. 44B.Furthermore, the external state of the display portion of the portableinformation terminal 9201 is a non-rectangular shape (a circular shapein FIG. 44B).

FIGS. 44C, 44D, and 44E are perspective views of a foldable portableinformation terminal 9202. FIG. 44C is a perspective view illustratingthe portable information terminal 9202 that is opened. FIG. 44D is aperspective view illustrating the portable information terminal 9202that is being opened or being folded. FIG. 44E is a perspective viewillustrating the portable information terminal 9202 that is folded.

The folded portable information terminal 9202 is highly portable, andthe opened portable information terminal 9202 is highly browsable due toa seamless large display region. The display portion 9001 of theportable information terminal 9201 is supported by three housings joinedtogether by hinges 9055. By folding the portable information terminal9202 at a connection portion between two housings 9000 with the hinges9055, the portable information terminal 9202 can be reversibly changedin shape from opened to folded. For example, the portable informationterminal 9202 can be bent with a radius of curvature of greater than orequal to 1 mm and less than or equal to 150 mm.

The display device which is one embodiment of the present invention canbe preferably used for the display portion 9001.

FIGS. 45A and 45B are perspective views of a display device 9500including a plurality of display panels. Note that the plurality ofdisplay panels are wound in the perspective view in FIG. 45A, and areunwound in the perspective view in FIG. 45B.

The display device 9500 illustrated in FIGS. 45A and 45B includes aplurality of display panels 9501, a hinge 9511, and a bearing 9512. Theplurality of display panels 9501 each include a display region 9502 anda light-transmitting region 9503.

Each of the plurality of display panels 9501 is flexible. Two adjacentdisplay panels 9501 are provided so as to partly overlap with eachother. For example, the light-transmitting regions 9503 of the twoadjacent display panels 9501 can be overlapped each other. A displaydevice having a large screen can be obtained with the plurality ofdisplay panels 9501. The display device is highly versatile because thedisplay panels 9501 can be wound depending on its use.

Moreover, although the display regions 9502 of the adjacent displaypanels 9501 are separated from each other in FIGS. 45A and 45B, withoutlimitation to this structure, the display regions 9502 of the adjacentdisplay panels 9501 may overlap with each other without any space sothat a continuous display region 9502 is obtained, for example.

The display device of one embodiment of the present invention can bepreferably used in the display panel 9501.

Electronic devices described in this embodiment are characterized byhaving a display portion for displaying some sort of information. Notethat the semiconductor device of one embodiment of the present inventioncan also be used for an electronic appliance that does not have adisplay portion.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 5

In this embodiment, the structure of a data processor including thedisplay device of one embodiment of the present invention will bedescribed with reference to FIGS. 46A and 46B.

FIG. 46A is a block diagram illustrating the structure of a dataprocessor 9600 including the display device of one embodiment of thepresent invention. FIG. 46B is a schematic diagram illustrating the dataprocessor 9600 being in operation.

The following describes components of the data processor 9600. In somecases, the components cannot be clearly distinguished from each otherand one component also serves as another component or includes part ofanother component.

<5. Structure Example of Data Processor>

The data processor 9600 includes an arithmetic device 9610 and aninput/output device 9620.

[Arithmetic Unit]

The arithmetic device 9610 includes an arithmetic portion 9611, a memoryportion 9612, a transmission path 9614, and an input/output interface9615.

[Arithmetic Portion]

The arithmetic portion 9611 has a function of executing a program.

[Memory Portion]

The memory portion 9612 has a function of storing a program executed bythe arithmetic portion 9611, initial information, setting information,an image, or the like. Specifically, a hard disk, a flash memory, amemory including a transistor formed using an oxide semiconductor, orthe like can be used as the memory portion 9612.

[Program]

A program is executed by the arithmetic portion 9611 through three stepsdescribed below with reference to FIG. 46B, for example.

In a first step, positional data P1 is acquired.

In a second step, a first region 9681 is determined on the basis of thepositional data P1.

In a third step, an image (image data Q1) with higher luminance than animage displayed on a region other than the first region 9681 is producedas an image displayed on the first region 9681.

For example, the arithmetic device 9610 determines the first region 9681on the basis of the positional data P1. The first region 9681 can have,specifically, an elliptical shape, a circular shape, a polygonal shape,a rectangular shape, or the like. A region within a 60-cm radius,preferably within a 5-30-cm radius, from the positional data P1 isdetermined as the first region 9681, for example.

To produce an image with higher luminance than an image displayed on aregion other than the first region 9681 as an image displayed on thefirst region 9681, the luminance of the image displayed on the firstregion 9681 is increased to 110% or more, preferably 120% or more and200% or less, of the luminance of the image displayed on the regionother than the first region 9681. Alternatively, the average luminanceof the image displayed on the first region 9681 is increased to 110% ormore, preferably 120% or more and 200% or less, of the average luminanceof the image displayed on the region other than the first region 9681.

As a result of the program, the data processor 9600 can generate theimage data Q1 with higher luminance than an image displayed on a regionother than the first region 9681 as an image displayed on the firstregion 9681 on the basis of the positional data P1. Consequently, thedata processor 9600 can have high convenience and can provide operatorswith comfortable operation.

[Input/Output Interface]

The input/output interface 9615 includes a terminal or a wiring. Theinput/output interface 9615 has a function of supplying data and afunction of receiving data. The input/output interface 9615 can beelectrically connected to the transmission path 9614 and/or theinput/output device 9620, for example.

[Transmission Path]

The transmission path 9614 includes a wiring. The transmission path 9614has a function of supplying data and a function of receiving data. Thetransmission path 9614 can be electrically connected to the arithmeticportion 9611, the memory portion 9612, or the input/output interface9615, for example.

[Input/Output Device]

The input/output device 9620 includes a display portion 9630, an inputportion 9640, a sensor portion 9650, and a communication portion 9690.

[Display Portion]

The display portion 9630 includes a display panel. The display panelincludes a pixel having a structure including a reflective displayelement and a transmissive light-emitting element. The luminance of adisplayed image can be increased by increasing the reflectance of thereflective display element or the luminance of the light-emittingelement with the use of the image data. That is, the display device ofone embodiment of the present invention can be preferably used in thedisplay portion 9630.

[Input Portion]

The input portion 9640 includes an input panel. The input panelincludes, for example, a proximity sensor. The proximity sensor has afunction of sensing a pointer 9682. Note that a finger, a stylus pen, orthe like can be used as the pointer 9682. For the stylus pen, alight-emitting element such as a light-emitting diode, a metal piece, acoil, or the like can be used.

As the proximity sensor, a capacitive proximity sensor, anelectromagnetic inductive proximity sensor, an infrared proximitysensor, a proximity sensor including a photoelectric conversion element,or the like can be used.

The capacitive proximity sensor includes a conductive film and has afunction of sensing the proximity to the conductive film. To determinepositional data, for example, a plurality of conductive films areprovided in different regions of the input panel and a region where afinger or the like used as the pointer 9682 approaches can be determinedin accordance with a change in parasitic capacitance of the conductivefilms.

The electromagnetic inductive proximity sensor includes a function ofsensing the proximity of a metal piece, a coil, or the like to a sensorcircuit. To determine positional data, for example, a plurality ofoscillation circuits are provided in different regions of the inputpanel and a region where a metal piece, a coil, or the like included ina stylus pen or the like used as the pointer 9682 approaches can bedetermined in accordance with a change in the circuit constant of theoscillation circuits.

The photo-detection proximity sensor has a function of sensing theproximity of a light-emitting element. To determine positional data, forexample, a plurality of photoelectric conversion elements are providedin different regions of the input panel and a region where alight-emitting element included in a stylus pen or the like used as thepointer 9682 approaches can be determined in accordance with a change inthe electromotive force of the photoelectric conversion elements.

[Sensor Portion]

As the sensor portion 9650, an illuminance sensor that senses theenvironmental brightness, a human motion sensor, or the like can beused.

[Communication Portion]

The communication portion 9690 has a function of supplying data to anetwork and acquiring data from the network.

The data processor 9600 described above can be used for education, orcan be used for a digital signage or a smart television system, forexample.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

This application is based on Japanese Patent Application serial no.2015-179114 filed with Japan Patent Office on Sep. 11, 2015, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a first displayelement; a second display element; a first transistor; and a secondtransistor, wherein the first display element comprises a first pixelelectrode and a liquid crystal layer, wherein the second display elementcomprises a second pixel electrode and a light-emitting layer, whereinthe first transistor is electrically connected to the first pixelelectrode, wherein the second transistor is electrically connected tothe second pixel electrode, wherein the first transistor and the secondtransistor each comprise an oxide semiconductor film, and wherein thefirst pixel electrode and the second pixel electrode each comprise atleast one metal element contained in the oxide semiconductor film. 2.The display device according to claim 1, wherein the first displayelement further comprises a reflective film, wherein the reflective filmis electrically connected to the first pixel electrode and has afunction of reflecting incident light, wherein the reflective film isprovided with an opening transmitting incident light, and wherein thesecond display element has a function of emitting light toward theopening.
 3. The display device according to claim 1, wherein the oxidesemiconductor film comprises In, Zn, and M, and wherein M is Al, Ga, Y,or Sn.
 4. The display device according to claim 1, wherein the oxidesemiconductor film comprises a crystal part, and wherein the crystalpart has c-axis alignment.
 5. The display device according to claim 1,wherein at least one of the first transistor and the second transistorhas a staggered structure.
 6. The display device according to claim 1,wherein at least one of the first transistor and the second transistorcomprise: a first conductive film, a first insulating film over thefirst conductive film; a first oxide semiconductor film over the firstinsulating film; a second insulating film over the first oxidesemiconductor film; and a second oxide semiconductor film over thesecond insulating film, wherein the second oxide semiconductor filmcovers a side surface of the first oxide semiconductor film with thesecond insulating film positioned therebetween in a cross section in achannel width direction, and wherein the first oxide semiconductor filmis surrounded by the first conductive film and the second oxidesemiconductor film in the cross section in the channel width direction.